California 

egional 

3-cility 


IMENT-MAKING 

OR  AMATEURS 


T~>    rj  i  f  \ 

. 


ELECTRICAL 

INSTRUMENT    MAKING 

FOR   AMATEURS. 

&  f  rsrffral  Ban&foHjk 

BY 

S.  R.  BOTTONE, 

LATE    OF    THE    COLLEGIO     DEL    CARMINE,     TURIN  J     THE    ISnTCTO 

BELLINO,  NOVAiA.     CERTIFICATED  SCIENCE  TEACHER, 

SOUTH  KENSINGTON. 


NEW  YORK: 
EXCELSIOR  PUBLISHING  HOUSE, 

29  AKD  31  BEEKMAN  STEBE-S?. 


oc 


PREFACE. 


THE  amateur,  especially  if  he  be  young,  inexperienced, 
and  unblest  with  "  filthy  lucre,"  must  ever  command 
our  sympathy  and  respect  He,  withput.  any  hope  of 
reward,  except  that  gratification  which  follows  the 
acquirement  of  knowledge,  works  on,  under  the  greatest 
difficulties,  to  the  attainment  of  his  end — an  end  which, 
in  nine  cases  out  of  ten,  tends  directly  to  the  benefit  of 
his  fellow-creatures. 

Nearly  all  the  really  useful  inventions  and  discoveries, 
which  have  rendered  the  igth  century  so  remarkable 
as  a  season  of  progress,  must  be  attributed  to  amateurs. 
For  this  reason,  if  for  no  other,  we  should  render  every 
assistance  in  our  power  to  the  'bond-fide  amateur,  and 
for  this  reason,  along  with  another,  have  I  attempted, 
in  the  following  pages,  to  guide  the  tyro  in  his  attempts 
at  the  construction  of  the  more  useful  pieces  of  electrical 
apparatus.  The  other  reason  is  that  "  a  fellow-feeling 
makes  us  wondrous  kind,"  and  as  I  myself  felt  acutely 
the  need  of  assistance  at  the  beginning  of  my  amateur- 
scientific  career,  so  I  am  pleased  to  suggest  when 


PREFACE. 

and  how  much  trouble  may  be  saved,  and  expense 
spared  by  the  adoption  of  certain  simple  modes  of 
procedure. 

In  the  following  pages  no  attempt  has  been  made 
to  describe  the  production  of  such  highly  finished 
"  brass  and  glass "  instruments  as  those  which  adorn 
the  windows  of  our  opticians'  shops.  Such  a  high 
degree  of  finish  requires  a  technical  knowledge  of 
French  polishing,  lacquering,  burnishing,  etc.,  as  is  not 
usually  possessed  by  the  amateur.  The  tools  used,  also, 
are  supposed  to  be  of  the  simplest  description,  such 
as  may  be  found  in  every  home,  however  humble. 
Not  one  of  the  instruments  described  necessitates  the 
employment  of  a  lathe  or  other  expensive  tool  in  its 
manufacture ;  though,  of  course,  much  truer  and  finished 
circular  work  can  be  done  on  the  lathe  than  in  any 
other  manner.  But  the  instruments  produced  as 
described  in  this  book,  may  be  relied  upon  to  act 
efficiently  ;  and  this  is,  after  all,  the  end  for  which  every 
instrument  is  'constructed.  It  must  be  borne  in  mind 
that  this  work  does  not  profess  to  teach  the  science  of 
electricity :  and  no  attempt  is  made  to  enter  upon  the 
domain  of  scientific  speculation, 


INDEX    TO    SECTIONS. 


SECTION                                              PACK 

SECTION                                              PAGE 

i  Tools      .... 

I 

72  Dynamos,     Rules    for 

2  Materials        ...        • 

2 

Winding  .         .        .  108 

3  Soldering       .                 . 

3 

72  Ring  Armature,    Paci- 

5          „        with  Flame    . 

7 

notti's        .         .         .  109 

6  Pivots     .... 

9 

72  Wires,  Resistances  of  .  112 

7  Glass  threads         .        . 

ii 

73  Resistance      of     Wire 

8  Straws    .... 

ii 

Table        .         .         .112 

9  Pith  Balls      . 

ii 

73  Wires,  Carrying  power  • 

10          „         Electroscope  . 

12 

of      .         .         .         .112 

ii  Gold  Leaf          „            . 

14 

74  Dynamos,  Winding     .  115 

12  Coulomb's       Torsion 

75  Ammeter      .         .         .116 

Balance 

20 

75  Ampere,  What  it  is      .116 

15  Electrophorus 

23 

79  Ammeter,  To  Grade    .   119 

17  Bertsch's  Machine. 

28 

80  Voltmeter    .         .         .121 

1  8  Mounting  a  Glass  Plate. 

29 

81  E.M.F.          .        .        .121 

20                  „                         „ 

82  Ohm's  Law  .        .        .122 

without  drilling  . 

34 

86  Galvanometers     .^        .131 

25  Carre's        Dielectric 

91  Tangents     and     Arcs,  " 

Machine     . 

40 

Table  of   .         .         .136 

29  Holtz's  Machine    . 

47 

93  Thermopile  .         .         .   139 

34  Wimhurst     „ 

57 

95   Batteries       .         ;         .   146 

42  Plate             „ 

65 

95  Porous  Cells         .         .  147 

43  Condensers    . 

70 

95  Binding    Screws,   and 

44  Leyden  Jar     . 

71 

Substitutes       .         .    148 

45  Franklin  Plate 

75 

95  Amalgamation     .         .148 

45   Fulminating  Panes 

75 

95  Zinc      ....  r48 

47  Fizeau's  Condenser 

76 

97  Graphite     Plates    and 

48  Microfarad       „      . 

79 

Rods  to  Mount.         .   150 

49  Medical  Coils 

81 

98  Batteries,  Single  Fluid.   151 

49  Shocking  Coil        .        . 

81 

98  Battery,      Agitating 

56  Induction  Coils      .        . 

88 

Fluid  in    .         .         .151 

58  Magneto  Machine. 

90 

99  Batteries,  Double  Fluid  153 

59      „    Shocking  „ 

90 

99          „         Table   of 

65       „    Uni-direction 

E.M.F.     .         .        .155 

Machine     . 

99 

lob  Telephone    .        .         .156 

72  Dynamos 

108 

102  Electro  Motors    .         .164 

72         „       Gramme. 

1  08 

103  The  Phonograph  .         .   170 

72        „       Manchester 

1  08 

104  The  Microphone.        .  180 

ELECTRICAL  INSTRUMENT  MAKING 
FOR  AMATEURS. 

§  I.  TOOLS. — The  true  amateur,  as  a  rule,  has  not  a 
large  assortment  of  tools.  Little  by  little  he  gets 
together,  or  constructs  those  which  are  necessary  for 
his  purpose ;  but  he  seldom  aspires  to  the  complete 
paraphernalia  of  a  workshop.  Still  there  are  certain  tooto 
that  are  indispensable,  of  which  the  following  is  a  lJs>t  In 
order  of  utility  : — 

i   Large  pocket-knife. 

I   Fine  penknife. 

I  Archimedean  drill  and  bits. 

Pair  of  cutting  pliers. 

Pair  of  large  scissors  for  metal. 

Pair  of  small  scissors. 

Several  files,  large  and  smalL  ' 

I  Hammer. 

I  Mallet. 

Bradawl,  gimlet,  pincers. 

Small  bench  vice. 

Small  tenon  saw. 

Soldering  iron. 

Spirit  lamp. 

I  Wheel  glass  cutter  or  diamond 

Pair  of  compasses 

aft.  rule,  s 

B 


^ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 

I  should  like  to  have  put  a  lathe  at  the  heacl  of  this 
list,  for  that  is  really  the  king  of  tools  ;  but  I  would  not 
deter  the  student  from  making  electrical  apparatus 
because  he  has  not  a  lathe,  as  most  may  be  made  well 
without,  though  better  with  one. 

Besides  tools,  the  materials  mentioned  below  will  be 
found  useful.  They  need  not  be  procured  all  at  once, 
but  as  occasion  demands.  If  the  amateur  adopts  the 
plan  of  keeping  up  a  little  stock  of  his  materials  and 
tools,  as  they  are  worn  out  or  consumed,  and  more 
especially  if  he  remembers  that,  "  Order  is  Nature's  first 
law,"  and  that  there  chould  be  "  a  place  for  everything, 
and  everything  in  its  place,"  he  will  turn  out  better  work, 
keep  his  temper,  and  v/ork  better  than  if  he  allows  him- 
self to  degenerate  into  a  slipshod  style  of  doing  things. 
Let  him  never  say  "  that'll  do  "  to  anything  capable  of 
improvement. 

\  2.  MATERIALS. — The  following  will  be  found  useful 
in  carrying  out  the  instructions  given  in  "the  ensuing 
pages  :— 

Glass  rods  from  J  to  \  in.  in  diameter. 

Ebonite  rods  from  Jto  \  in.  in  diameter. 

Glass  tubes  from  J  to  i  in!  in  diameter. 

Guttapercha. 

Glass  bottles,  preferably  green  glass. 

Sheets  of  glass  ;  every  piece  is  useful. 

Bottoms  of  broken  wine-glasses  as  stands,  &c. 

Tinfoil 

Sheet  zinc  and  sheets  of  tinned  iron.* 

*  Clean  beef  tins,  sardine  tins,  &c.,  may  be  worked  up  very  well  and 
econouvically, 


MATERIALS.  $ 

Sheet  copper. 

Sheet  brass,  and  brass  rod,  iin.  diameter* 

Solder. 

Chloride  of  zinc.. 

Rosin. 

Needles. 

Watch  springs. 

MVhite  hard  varnish. 

Red  lead. 

Benzoline. 

Burnt  umber. 

Copper  wire  of  various  sizes.* 

Prout's  elastic  glue. 

Methylated  spirits  of  wine. 

Having  these  materials  at  hand,  the  amateur  will  find 
several  operations  are  required  so  frequently  as  to  render 
a  certain  amount  of  technical  skill  absolutely  necessary 
if  the  work  is  to  look  neat  and  act  satisfactorily.  Among 
the  first  of  the -amateur's  requirements  must  be  placed 
the  power  of  soldering. 

§  3.  SOLDERING. — For  small  work,  an  iron,  shaped  as 
shown  at  Fig.  i,  will  be  found  extremely  useful.  The 


FIG.  i. 

amateur  can  easily  construct  this  for  himself  by  boring 
a  tff  hole  in  a  copper  wedge  2in.  long  by  Jin.  thick,  and 
£in.  wide  on  the  base.  An  iron  rod,  15in.  long,  is 
straightened  out  for  the  handle,  and  the  end  of  this  rod 

.  Thia  had  better  tie  procured  ^s  required. 


4  ELECTRICAL  INSTRUMENT  MAKING  FOR'AMATEURS* 

is  heated  in  the  fire  and  hammered  up  until  it  can  be  thrust 
through  the  hole  in  the  copper  wedge;  what  projects  is 
then  hammered  down  so  as  to  form  a  kind  of  boss  or  rivet 
head,  to  prevent  the  copper  bit  from  moving.  The  other 
end  of  the  rod  may  then  be  filed  or  hammered'to  a  point 
and  driven  into  a  wooden  handle.  Five  inches  of  broom- 
handle  will  answer  the  purpose,  if  nothing  better  can  be 
got  at 

In  order  to  solder  successfully,  four  things  are 
essential.  Firstly,  the  portions  to  be  soldered  must  be 
made  scrupulously  clean,  either  by  scraping  with  a  p\ece 
of  sharp  knife  blade  (kept  specially  for  this  purpose),  or 
by  sandpapering,  or  by  filing,  whichever  is  most 
convenient.  Secondly  the  portions  that  are  to  be 
soldered  must  be  raised  nearly  to  the  temperature  of  the 
melted  solder.  For  this  reason  the  amateur  would 
surely  fail  were  he  to  attempt  to  make  a  good  joint,  say, 
Between  two  sad  irons,  with  an  ordinary  bit,  unless  he 
previously  heated  the  irons  to  nearly  the  melting  point 
of  solder.  In  ordinary  small  work,  where  wires  are 
soldered  to  wires,  sheets  to  sheets,  &c.,  the  heat  of  the 
soldering  iron  itself  is  generally  sufficient,  not  only  to 
melt  the  solder,  but  also  to  raise  the  temperature  of  the 
surrounding  portions  to  be  joined  to  the  requisite  point 
Thirdly,  the  nose  of  the  soldering  iron  must  be  perfectly 
clean,  and  covered  with  melted  solder,  or,  as  it  is  tech- 
nically called  "  tinned." 

To  insure  this  result  the  soldering  iron  must  be  placed 
in  a  clear  red  fire  until  nearly  red-hot  (if  allowed  to  get 
too  hot  it  will  not  take  up  the  solder  at  all),  then  quickly 


SOLDERING.  5 

and  lightly  filed  on  one  face,  and  at  once  plunged  into  a 
rod  of  soft  solder,  on  which  a  little  powdered  rosin  has 
been  placed  The  heat  of  the  soldering  iron  will  cause 
the  solder  to  melt,  and  run  into  globules.  If  one  of 
these  be  allowed  to  run  into  the  lid  of  a  milk  tin,  or 
other  convenient  tinned  iron  receptacle  in  which  a  little 
powdered  rosin  is  also  placed,  and  the  filed  face  of  the 
soldering  iron  rubbed  briskly  over  the  melted  solder,  the 
surface  of  the  copper  bit  will  be  found  to  have  taken  a 
coating  of  solder  as  brilliant  as  a  looking-glass.  Should 
this  not  be  the  case,  the  iron  must  again  be  heated  and 
filed  and  rubbed  over  the  solder  until  a  perfect  coating 
has  been  obtained. 

When  once  the  nose  of  the  copper  bit  is  well  "tinned," 
it  is  sufficient  after  each  heating  (provided  it  has  not 
been  overheated  so  as  to  burn  off  the  solder)  to  rub  the 
iron  briskly  on  a  piece  of  old  carpet,  kept  especially  for 
this  purpose,  just  before  touching  the  solder.  Fourthly, 
the  surfaces  to  be  united  must  be  kept  perfectly  clean, 
and  unoxidised  during  the  application  of  the  heat 
necessary  for  soldering,  by  the  application  of  some 
substance  which  prevents  the  access  of  air,  &c.,  to  the 
heated  surfaces. 

These  substances' are  various,  and  some  are  more 
adapted  to  one  metal  than  to  another.  Thus  rosin  is 
excellent  for  tinned  iron  and  for  copper.  Hydrochloric 
acid  (spirits  of  salt)  is  perhaps  the  best  for  zinc. 
Chloride  of  zinc  (killed  spirits  of  salt),  again,  is  excellent 
for  iron,  for  copper,  and  for  brass,  where  the  surfaces  can 
afterwards  be  well  washed. 


6  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 

A  lump  of  sal-ammoniac  (chloride  of  ammonia)  is  also 
very  useful  for  removing  the  oxidation  from  the  copper 
tint  by  rubbing  it  against  the  lump  for  a  few  seconds  after 
heating. 

§  4.  As  an  example  of  the  mode  of  proceeding,  let  us 
suppose  we  wish  to  solder  a  wire  to  the  copper  plate  of  a 
Daniell  battery.  We  begin  by  cleaning  the  copper  plate 
at  the  spot  to  which  we  wish  to  attach  the  wire,  by 
rubbing  it  with  a  piece  of  glass  or  sandpaper  until  the 
surface  is  as  brilliant  as  a  mirror.  In  like  manner  we 
polish  and  clean  the  end  of  the  copper  wire.  Laying  the 
copper  plate  flat  on  the  board  which  we  keep  expressly 
for  soldering  on,  we  place  the  copper  wire  on  the  desired 
spot. 

We  now  put  the  soldering  iron  into  the  fire*  (it  having 
been  previously  tinned  as  described),  and  watch  it  until 
it  shows  by  the  melting  of  the  solder  on  the  surface 
that  it  is  hot  enough.  We  then  remove  it  from  the  fire, 
give  a  rub  on  the  sal-ammoniac  or  piece  of  carpet,  then 
take  up  a  globule  of  solder  by  touching  one  with  the" 
cleaned  nose  of  the  bit,  and,  lastly,  having  quickly 
touched  the  surfaces  both  of  the  copper  plate  and  wire 
with  a  feather  dipped  into  the  chloride  of  zinc  solution, 
rub  the  surface  of  the  plate  and  wire  simultaneously  with 
the  soldering  iron.  As  soon  as  the  surfaces  become 
sufficiently  heated,  the  solder  will  be  seen  to  flow  over 
them.  The  wire  must  then  be  pressed  into  its  desired 
place,  the  melted  solder  rubbed  well  over  the  point  of 

Care  must  be  taken  that  the  fire  is  emitting  no  sulphurous  smoke, 
otherwise  the  mm  will  surely  not  take  up  the  solder. 


SOLDERING  WITH  A  FLAME.  7 

junction,  the  iron  removed,  while  the  wire  is  held  motion- 
less in  its  position,  until  a  sudden  dulling  of  the  surface 
of  the  solder  shows  that  it  has  set  and  is  solid  enough  to 
hold  the  parts  together.  After  soldering  with  chloride 
of  zinc  ("  killed  spirits,"  "  soldering  fluid  "),  always  wash 
in  plenty  of  water  to  prevent  rusting. 

§  5.  SOLDERING  WITH  A  FLAME.— In  many  instances 
a  better  joint  and  neater-looking  work  can  be  made  over 
the  flame  of  a  spirit  or  other  lamp  than  with  the 
soldering  iron.  Thte  is  more  especially  the  case  in 
small  Work,  such  as  joining  wires,  soldering  pivots,  &c 
As  an  example,  let  us  suppose  we  desire  to  make  a 
poised  magnetic  needle  out  of  two  similar  pieces  of 
needle,  joining  them  together  by  means  of  a  short 
tinned-iron  junction,  in  which  the  pivot  is  inserted.  (It 
is  evident  that  a  pivot  could  not  well  be  attached  to  an 
ordinary^  sewing  needle  were  the  needle  in  one 
piece.) 

The  needles  (of  which  two  are  required)  are  broken 
off  of  the  desired  length  by  means  of  a  pair  of  nippers. 
The  heads  may  be  the  portions  rejected,  if  a  very  light 
needle  is.  required  ;  the  points,  if  a  heavier  needle  be  net 
objectionable.  Care  should  be  taken  that  the  pieces  be 
of  the  same  weight,  to  insure  a  well-balanced  needle. 
'A  small  piece  of  sheet  tinned  iron  ("  tin-plate ")  about 
1  in.  square  should  now  be  procured  and  flattened  out. 
With  a  screw-drill,  or  small  punch,  a  clean  central  hole 
is  made  a  trifle  smaller  than  the  largest  external 
diameter  of  the  pivot  (See  §  6  for  pivots.)  The  piece 
js  now  cut  into  the  shape  of  a  small  lozenge,  as  shov/i* 


8  ELECTRICAL  INSTRUMENT  MAKING  FOR-AMATEURS, 

at  Fig.  2,  A,  and  again  flattened  out  by  a  light  blow 
with  a  flat-faced  hammer.  The  needles  are  now  to  be 
lightly  sandpapered  at  the  ends  which  r  are  to  be 
soldered  to  the  lozenge,  then  these  extremities  immersed 


Fio.  a. 

in  the  chloride  of  zinc  soldering  fluid.  The  soldering; 
iron  is  then  to  be  heated,  and  a  little  globule  of  solder 
caused  to  adhere  to  the  prepared  extremity  of  each 
needle  by  its  aid.  The  needles  should  now  present  the 


SOLDERING .  WITH  A  FLAME.  $ 

appearance  shown  in  Fig.  2,  B.  The  spirit  lamp  is  now 
to  be  brought  into  requisition.  The  flame  should  not 
be  too  high  for  such  work,  so  that  the  wick  need  not  be 
raised  much.  Taking  up  the  lozenge-shaped  piece  in 
the  left  hand,  with  a  pair  of  tweezers  (or  longish  piece  of 
wire  bent  to  a  tweezer  shape),  and  allowing  one  half  to 
project,  we  apply  with  a  feather  a  little  soldering  fluid  to 
the  projecting  point  The  needle ,  with  its  adhering 
solder  is  now  taken  up  in  the  right  hand  (a  small  piece 
of  paper  being  wrapped  round  it,  to  prevent  the  fingers 
being  burnt),  and  both  the  needle  and  the  lozenge  are 
held  for  a  few  seconds  over  the  flame  of  the  spirit  lamp,1 
in  the  position  they  are  desired  to  remain  (See  Fig/ 2,  c). 
The  solder  will  soon  be  seen  to  flow.  As  soon  as  this 
is  the  case  the  pieces  must  be  pressed  firmly  together,3 
and  both  hands  removed  from  over  the  flame  of  the 
lamp,  care  being  taken  to  keep  the  parts  constantly 
pressed  together  4in til  a  sudden  dulling  shows  that  the 
solder  is  set.  In 'like?  manner  the  other  needle  is 
soldered  to  the  other  point  of  the  lozenge.  The  needle 
may  then  be  magnetised,  and  after  magnetisation 
balanced  on'  its  pivot,  as  will  be  described  further  on. 
Fig.  2,"D,  shows  the  needle  ready  for  pivoting. 

§  6.- PIVOTS. — These  are*  in  !  constant  requisition; 
consequently,  it  is  well  to  be  able  to  make  them  of  all 
sizes.  The  amateur  will  need  only,  two  kinds — viz, 
in  brass  and  in  glass.  Brass  pivots  are  very  easily  made 
from  stout,  hard,  brass  wire.  The  most  useful  size  is 
about  Ath  in.  in  diameter.  A  short  length  of  this  is 
placed  in  the  jaws  of  a  vice,  with  Jts_circular  section 


10  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 

uppermost  A  bit,  capable  of  cutting  a  i*«th  hole, 
is  placed  in  the  screw-drill,  and  after  being  oiled,  the 
drill  is  worked  over  the  brass  rod,  the  bit  resting  quite 
centrally,  until  the  shoulders  of  the  bit  just  touch  the  out- 
side of  the  wire.  This  produces  a  good  deep  and  evenly 
conical  hole.  The  wire  can  now  be  removed  from  the  vice, 
and  the  outside  of  the  pivot  also  made  conical  by  filing, 
while  still  on  the  brass  rod  ;  when  the  desired  shape  has 
£>een  secured,  it  cam  be  cut  off  with  a  fret-saw  or  file. 

Glass  pivots  are  extremely  useful  in  all  electrical 
experiments.  The  best  way  to  make  these  is  to  soften 
a  glass  tube  over  a  spirit  lamp,  and  gradually  but 
steadily  pull  it  asunder.  By  so  doing,  two  conical 
pieces  are  the  result  If  the  extreme  hair-like  points  of 
these  are  held  over  the  lamp,  they  fuse  up  into  a  round 
globule,  thus  closing  up  the  hole.  The  pivots  thus 
produced  can  easily  be  cut  off  (when  the  glass  is  cold.)  by 
making  a  mark  round  the  tube  with  a  sharp  triangular 
file,  at  the  point  where  it  is  desired  to  break  it  off.  On 
applying  a  little  pressure  between  the  finger  and  thumb 
of  both  hands,  the  pivot  easily  breaks  away  from  the 
remainder  of  the  tube.  The  pivots  may  be  made  very 
long  and  thin,  by  allowing  the  glass  to  soften  well,  before 
beginning  to  pull  asunder,  and  such  long  thin  pivots  are 
very  useful  for  astatic  galvanometers,  or  other  cases  in 
which  two  objects  have  to  be  poised  on  one  pivot  Short 
pivots  can  be  made  by  pulling  asunder  as  soon  as  the 
glass  softens  in  the  flame.  Fig.  2,  C,  shows  how  the  glass 
tube  should  be  held  in  the  hands  to  soften,  over  the  flame. 
Fig.  2,  E,  gives  the  appearance  of  the  tube  after  pulling 


CLASS  THREADS.  SI 

asunder.  Fig.  2,  F,  indicates  how  the  hair-like  extremity 
should  be  melted  into  a  bead  over  the  lamp,  so  as  to  close 
the  capillary  aperture,  and  Fig.  2,  G,  shows  finished  pivots. 

§  7.  GLASS  THREAD^ — These  are  extremely  useful 
as  insulators  in  small  work,  such  as  electroscopes, 
carriers,  torsion  balances,  &c.  They  are  easily  made 
from  rather  thin  glass  rod,  by  heating  it  carefully  over  a 
clear  flame  (spirit-lamp,  or  Bunsen  burner)  until  soft,  and 
then  pulling  asunder  rapidly  if  a  very  fine  thread  be 
desired  ;  more  slowly,  if  a  coarser  filament  be  required. 
Similar  threads  may  be  made  from  shellac  ;  and  these, 
though  somewhat  more  brittle,  are  even  better  insulators. 

§  8.  STRAWS. — The  straws  of  various  grasses,  more 
especially  the  fine,  straight,  hair-like  terminals  of  such 
grasses  as  Agrostis  spicavzntit  Alopecnrus  pratensis, 
Phalaris  arundinacca,  and  Aira  cristata,  if  gathered  soon 
after  the  flowers  are  fully  matured,  cut  to  the  length  of 
about  6  inches,  dried,  and  then  boiled  in  melted  paraffin 
wax,  make  excellent  insulating  supports,  far  stronger  than 
the  glass  or  shellac  ones  mentioned  above.  The  same 
straws,  not  paraffined,  are,  from  their  lightness,  well 
adapted  as  pointers  for  such  galvanometers  as  tangents 
and  others,  which  require  a  separate  indicator. 

§  9.  PlTH-BALLS. — These  are  much  used  in  experi- 
ments with  frictional  electricity.  The  best  piths  for 
ordinary  purposes  are  those  of  the  elder,  Sambucus  nigra. 
and  of  the  Jerusalem  artichoke,  Helianthus  tuberosust 
Before  being  shaped  into  balls  or  other  figures,  the  pith 
should  be  thoroughly,  dried.  With  a  penknife,  reduced 
to  a  razor-like  sharpness,  the  pith  can  be  cut  to  any 


12  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 


desired  figure,  and  is  easily  rounded  to  an  approximate 
sphere.  When  a  number  of  equal  size  have  been  made, 
the  final  rounding  may  be  given  by  lightly  rolling  them 
with  a  smooth,  flat  board,  on  a  level  table.  Care  must 
be  taken  not  to  press  too  heavily,  otherwise  the  balls 
will  be  flattened.  Having  now  got  together  the  few 
things  necessary  for  starting  work,  we  may  try  our  con- 
structive abilities  on  the  simpler  forms  of  electroscopes. 

Electroscopes  are  instruments  employed  for  the 
detection  of  the  presence  (and  sometimes  of  the  nature) 
of  electricity ;  not  for  its  measurement 

$  10.  PITH-BALL  ELECTROSCOPES.— These  are  of  two 


FJG.  3. 

kinds— viz.,  those  in  which  the  balls  are  suspended  by  a 
filament,  and  those  in  which  the  suspension  consists  of 
a  horizontal  rod,  pivoted  at  the  centre.  Fig.  3,  A,  shows, 
the  former  ;  B,  the  latter  form. 


PITH-BALL  ELECTROSCOPES.  13 

To  make  the  former,  a  glass  rod  about  Sin.  long  by 
Jin.  thick,  should  have  one  extremity  heated  over  a 
spirit  lamp,  and  when  quite  soft,  touched  with  another 
piece,  so  as  to  enable  the  operator  to  draw  out  a  filament, 
which  must  be  bent  into  the  shape  of  a  recurved  hook. 
The  rod  must  then  be  heated  at  a  point  about  2in.  from 
this  end,  and,  when  sufficiently  softened,  bent  neatly 
(not  too  hurriedly)  at  right  angles.  The  bent  rod  must 
be  allowed  to  cool  gradually  without  touching  anything, 
otherwise  it  will  be  liable  to  fly  to  pieces.  When  quite 
cold  it  may  be  fastened  to  the  foot  of  a  broken  wine- 
glass by  means  of  a  turn  or  two  of  stout  brown  paper, 
previously  brushed  over  with  good  Russian  glue,  as 
shown  at  C.  When  this  is  quite  dry  it  should  receive  (at 
this  joint  only)  a  coating  of  red  varnish,  made  by  mixing 
a  few  grains  of  red  lead  with  a  teaspoonful  of  white 
hard  varnish.  (The  white  hard  varnish  may  be  pro- 
cured at  any  oilman's.)  A  single  filament  of  cocoon  silk 
is  now  to  be  procured,  and  a  fine  needle  threaded  with 
it  The  needle  is  passed  through  the  centre  of  a  pith- 
ball,  the  end  of  silk  fibre  moistened  with  a  drop  of  glue, 
and  the  needle  pulled  until  the  ball  reaches  the  glued 
portion  of  the  silk.  This  will  serve  to  fasten  it  to  the 
b  ilL  The  other  end  of  the  fibre  may  be  tied  or  glued  to 
the  little  hook  of  the  bent  glass  arm.  Two  balls  may  be 
thus  suspended,  if  it  is  required  to  show  the  repulsive 
effects  of  similarly  charged  bodies.  To  construct  the  hori- 
zontally-pivoted form,  it  is  only  necessary  to  split  a 
straight  paraffined  straw  (§  8)  at  its  centre,  with  a  sharp 
penknife,  insert  a  small  glass  pivot  in  the  split,  and 


14  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 

fasten  it  thereto  by  means  of  a  single  drop  of  hot  glue. 
When  quite  cold,  a  pith-ball  must  be  attached  to  each 
end  of  the  straw,  a  small  hole  being  made  in  each  ball 
with  a  pin,  and  the  end  of  the  straw  (previously  touched 
with  glue)  inserted  in  the  hole*  Care  must  be  taken 
at  this  point  that  the  balls  balance  one  another.  They 
may  be  made  to  do  this  by  sliding  alo'ng  the  straw,  until 
when  placed  on  a  needle-point  the  arms  of  the  electro- 
scope remain  perfectly  level.  Half  of  an  ordinary  sewing- 
cotton  reel  maybe  used  as  the  foot  of  this  electroscope. 
After  sawing  in  half,  the  upper  portion  should  be  filed, 
rounded,  and  smoothed,  a  short  length  (say4in.)  of  cane, 
glued,  and  thrust  in  the  central  hole ;  a  needle  (point  up- 
wards) forced  into  the  upper  extremity  of  the  cane  ; 
and,  lastly,  the  whole  wooden  portion  neatly  varnished 
with  the  red  varnish  as  described. 

§  ii.  GOLD  LEAF  ELECTROSCOPE.— This  is  a  most 
useful  instrument  for  the  detection  of  minute  charges  of 
electricity.  If  well  made,  it  also  serves  admirably  to 
show  the  phenomena  of  induction.  The  requisites  are 
a  tall  wide  glass  jar,  a  sheet  of  gold  leaf,  a  couple  of 
strips  of  tinfoil,  a  short  piece  of  brass  rod,  about 
i-inch  in  diameter,  a  couple  of  beef  tin  bottoms,  or 
similar  pieces  of  tinned  iron,  and  an  empty  cigar 
box. 

For  the  glass  jar,  we  may  either  use  the  straight  glass 
chimneys  (about  3  inches  in  diameter  by  7  inches  in 
height),  that  are  used  for  large  gas  or  paraffin  burners, 
and  which  may  be  obtained  at  any  respectable  iron- 
mongers for  a  few  pence  (see  Fig.  4,  A),  or  the  bell- 


GOLD  LEAF  ELECTROSCOPE.  jj 

shaped  chimneys  (see  Fig.  4,  B).     Whichever  shape  be 
selected,  the  diameter  should  not  be  less  than  3  inches 


FIG.  4. 


nor  more  than  4  inches.     Preference  should  be  given  to 
the  bell  form,  when  procurable^  as  the  "  cap  "  is  more 


tf  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 

easily  fitted  to  the  narrow  neck.  Having  procured  a 
glass  jar,  the  next  step  is  to  fit  a  cover  to  both  extremi- 
ties. If  Fig.  R  be  chosen,  the  wider  extremity  which  Is 
'destined  to  form  the  bottom,  should  be  fitted  with*  a 
tinned-iron  cover.  If  the  bottom  of  the  glass  be 
3  inches  diameter,  it  will  be  fitted  exactly  by  the  bottom 
of  a  Swiss  milk  tin.  The  bottom  must  not  fit  too  tightly, 
if  fitted  in  winter,  or  else  the  slightest  increase  in  tem- 
perature will  expand  the  glass  and  cause  it  to  crack.  If 
a  tin  "  bottom  "  is  not  at  hand  to  fit,  it  may  readily  be 
made,  by  striking  out  on  a  piece  of  thin  tinned-iron,  a 
circle  of  the  same  size  as  the  outside  of  the  glass  jar. 
Keeping  about  £-inch  outside  this  line,  the  circle  is  cut 
out,  and  then  the  edge  turned  up  square  by  hammering 
lightly  on  the  edge  of  an  ordinary  iron.  The  tin  disc 
should  now  show  ^he  appearance  sketched  at  Fig.  4,  C. 
This  bottom  must  not  be  fastened  in  until  after  the  gold 
leaves  have  been  adjusted.  The  next  operation  consists 
in  making  the  cover  to  fit  the  other  end  of  the  jar  or 
cylinder.  If  A  has  been  chosen,  two  circular  discs 
must  be  cut  out  of  the  cigar  box  wood ;  one  that  will 
just  drop  into  the  jar,  the  other  precisely  the  diameter 
of  the  outside  of  the^ar.  The  two  circles  had  better  be 
measured,  struck  out  with  the  compasses,  and  sawn  with 
a  fret  or  band-saw.  The  two  discs,,  after  being  well 
smoothed,  should  be  glued  together,  as  shown  in  Fig. 
4,  D.  If  B  is  our  choice,  half  a  sewing-machine  cotton 
reel  will  Be  found  to.  fit  with  a  trifling  adjustment.' 
All  that  will  be  necessary  in  this  case  -will  be  to  cut  of? 
the  narrow  portion,  leaving  one  of  the  coned  heads,  the 


COLD  LEAF  ELECTROSCOPE.  17 

rim  of  whicli  can' be  trimmed  with  a  rasp,  until  it  just 
fits  the  small  mouth  of  the  chimney.  Whichever  form 
be  adopted,  this  wooden  cover  must  have  in  it  two  per- 
forations, one  central,  about  J  inch  in  diameter,  wherein 
will  pass  the  brass  rod  from  which  depend  the  gold 
leaves  ;  the  other  near  the  circumference,  which  will 
serve  as  an  exit  for  moist  air.  Fig.  4,  E  shows  the  glass 
chimney  fitted  with  its  lower  "  tray  "  and  upper  "  cap." 
At  exactly  opposite  diameters  of  the  glass  jar,  two 
strips  of  tinfoil,  about  |  inch  wide  by  4  inches  long,  are -to 
be  pasted  itisidt  the  jar,  reaching  from  a  little  above 
the  middle  downwards  to  the  metal. tray.  The  object  of 
these  is  twofold  ;  they  increase  the  sensibility  of  the  in- 
strument by  heightening  the  apparent  repulsive  effect  of 
a  charge ;  and  they  serve  to  discharge  the  gold  leaves 
without  tearing,  if  by  inadvertence  an  overcharge  be 
given.  This  being  done,  the  brass  rod  may  be  fitted  to 
its  table.  Maruy  operators  use  a  brass  ball  at  the  top  of 
the  gold  leaf  electroscope ;  for  general  purposes,  a 
metallic  table  will  be  found  more  sensitive  and  more 
convenient.  Having  cut  a  tinned-iron  or  thin  brass  disc 
3^  inches  in  diameter,  let  the  edges  be  turned  in  by  ham- 
mering. To  the  centre  of  this  metal  disc,  and  perpendi- 
cular to  it,  should  be  soldered  about  6  inches  of  brass 
rod,  having  a  small  hole  drilled  in  it  at  half  its  length 
— say  iinch  in  diameter.  Care  must  be  taken  that 
this  rod  is  quite  straight  and  smooth.  The  under  por- 
tion of  the  disc,  as  well  as  the  rod  to  within  \  inch  of 
its  extremity,  should  now  be  varnished  with  red  varnish 
(see  §  io),and  set  aside  to  dry.  While  this  is  drying, 


iS  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 

the  upper  wooden  cap  of  the  cylinder  may  be  immersed 
in  melted  paraffin  wax,  and  allowed  to  remain  therein 
until  thoroughly  permeated  with  paraffin.  The -brass 
rod  may  now  be  passed  into  the  central  orifice  of  the 
"cap," and  tightly  fitted  thereto  by  means  of  a  roll  or 
two  of  paraffined  brown  paper.  (N.B. — When  paraffin 
is  mentioned  in  this  work,  paraffin  wax  is  understood, 
not  paraffin  oil.}  The  rod  must  be  pushed  down  in  the 
orifice  until  the  J-inch  hole  is  just  level  with  the  top  of 
the  cap.  A  short  brad,  or  similar  piece  of  wire,  pushed 
forcibly  into  this  hole,  will  effectually  prevent  the  rod 
sinking  through,  if.  any  weight  be  afterwards  placed  on 
the  table  above.  A  small,  flat  .piecet  of  brass,  about 
\  inch  long  by  £  inch  square  section,  must  now  be 
soldered  to  the  lower  end  of  the  rod,  transversely  to  it, 
so  as  to  form  a  letter  JL-  It  is  to  this  transverse  piece 
that  the  gold  leaves  are  to  be  attached.  The  gold  leaves 
should  be  about  £  ineh  wide  by  about  2  inches  long. 
To  cut  them  neatly  is  not  an 'easy  job  for  the  tyro ;  still, 
it  may  be  managed  in  the  following  manner : — > 

Having  procured  a  clean  half-sheet  of  note  paper,  let 
it  be  folded  down  the  middle.  This  is  to  be  placed  open 
close  by  the  side  of  the  book  containing  the  gold  leaf. 
Holding  his  breath  for  a!  few  seconds,  so  as  not  to  blow 
away  the  leaf,  the  operator  slides  the  edge  of  the  note 
paper  under  one  of  the  leaves  of  gold,  and  assisting 
operations  with  a  pin  point,  or  perfectly  clean  camel- 
hair  pencil,  coaxes  the  gold  leaf  so  that  it  lies  flat  and 
square  in  the  centre  of  the  note  paper.  He  then  covers 
it  over  with  the  bent  half  of  the  note  paper,  so  that  the 


GOLD  LEAF  ELECTROSCOPE.  ig 

gold  leaf  lies  between  the  two  leaves  of  paper,  forming  the 
half-sheet  of  note.  Then,  with  a  pair  of  perfectly  clean 
sctesors,  he  cuts  the  paper  into  strips,  about  \  inch  wide 
by  2  inches  long.  In  so  doing,  he  must  take  care  not 
to  let  the  papers  separate,  otherwise  his  gold  leaves  will 
get  crumpled,  or  blow  about.  Of  course,  in  cutting  the 
paper  he  cuts  the  gold  leaf  into  the  desired  size.  He 
then  touches  the  two  sides  of  the  "transverse  piece  of  brass 
(attached  to  the  rod  passing  through  the  cap  of  the 
electroscope)  with  the  merest  trace  of  gum,  then  care- 
fully lifting  the  upper  pieces  of  paper  off  the  strips  of 
gold  leaf,  takes  up,  first  on  one  side,  and  then  on  the 
other  (of  the  transverse  piece)  a  strip  of  gold  leaf,  being- 
careful  that  they  hang  straight  down  and  parallel 
Having  succeeded  in  getting  the  two  strips  to  hang 
pauarely  and  free  from  one  another,  the  operator  next 
inserts  *hem  very  cautiously  into  the  chimney.  The 
cap  may  now  be  cemented  down  on  to  the  neck  of  the 
chimney  by  means  of  a  piece  of  silk  ribbon  moistened 
with  thin  glue.  When  this  is  quite  dry,  and  after  the 
bottom  has  been  likewise  cemented  to  the  lower  end  of 
the  chimney  or  cylinder,  a  coating  of  red  varnish  is 
applied,  care  being  taken  not  to  stop  up  the  side  air- 
hole. This  air-hole  should  be  fitted  with  a  small  wooden 
plug,  furnished  with  a  rounded  knob,  likewise  varnished. 
A  small  quantity  of  Prout's  elastic  glue,  run  round  the 
edge  of  the  warmed  tin,  will  be  found  the  most  effectual 
way  of  fastening  the  bottom  to  the  lower  portion  of  the 
glass  chimney  or  cylinder.  Care  must  be^  taken  that  the 
two  tinfoil  strips  are  opposite  the  width  of  the  gold  leaves. 


20  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 

and  that  the  said  strips  make  metallic  contact  with  the 
tin  bottom.  If  all  has  been  carefully  executed,  the  elec- 
troscope will  present  the  appearance  shown  at  Fig.  F, 
and  will  be  so  delicate  as  to  give  a  large  divergence  of 
its  leaves,  if  a  rubbed  rod  of  sealing-wax  is  held  at  a 
distance  of  one  foot  from  the  "  table,"  or  upper  plate. 

§12.  COULOMB'S  TORSION  BALANCE. — Apart  from 
the  actual  use  of  this  instrument  as  an  accurate  measurer 
of  electric  and  magnetic  force,  it  is  extremely  service- 
able in  calculating  the  laws  of  electrical  attraction,  and 
repulsion,  viz.,  that  these  are  "inversely  as  the  squares, 
of  the  distance,  and  directly  as  the  charges." 

To  make  such  an  instrument,  we  select  a' glass  chimney 
similar  to  that  shown  in  our  last  section;  Fig.  4,  A. 
This  must  be  fitted  with  a  .metal  top,  precisely  like  that 
described  for  the  bottom  'cover  of  the  electroscope, 
Fig/4,  C.'^In  the  centre  of  this  a  small  aper- 
ture is  made  to  admit  of  the  introduction  of  about 
I  inch  of  thin  brass  tube,  having  about  £  inch  bore.  This 
is  soldered  neatly  into  the.c"over,  so  that  the  tube  pro- 
jects about  \  inch  on  either  ^ide  of  the  cover.  A  block 
of  mahogany  -or  deal,  about  4  inches  square  by  £  inch 
thick,  is  now  planed  up  and  made  truly  square.  A  cjr- 
£ular  channel,  about  \  inch  wide,  and  of  the  same  depth, 
is  now  cut  in  this  base  board  for  the  glass  to  stand  in. 
If  deal,  this  base  should  be  stained  black  and  varnished ; 
if  mahogany  it  should  be  polished. 

\  13.  An  efficient  black  stain  for  such  pieces  of  white 
wood  may  be  made  by  working  upon  a  slab  a  teaspoonful 
Df  lamp-black,  with  a  tablespoonful  of  thin  glue,  until 


COULOMB'S  TORSION  BALANCE.  « 

quite  smooth,  with  a  muller.  This  stain  may  be  applied 
while  warm  to  the  wood,  well  rubbed  in,  and  when  quite 
dry  varnished  with  "  white  hard  varnish,"  which  will  be 
dry  in  about  twelve  hours. 

§  14.  A  circular  card,  graduated  to  the  360*  of  a  circle, 
and  of  the  same  diameter  as  the  interior  of  the  glass 
chimney,  is  now  to  be  glued  on  to  the  central  circle  of 
the  base  board  ;  the  zero  point  coinciding  with  the 
centre  of  one  of  the  flat  edges,  not  with  the  diagonal 
corners.  Previous  to  gluing  down  the  card,  a  slot  about 
|  in.  deep,  and  \  in.  wide,  must  be  cut  in  the  upper  surface 
of  the  base  board,  reaching  from  the  position  of  the  zero 
point  to  the  extreme  edge  of  the  board.  (This  serves  for 
the  introduction  of  a  soft  iron  rod,  or  of  different 
magnets.)  The  next  step  is  to  make  a  similar  but 
rather  smaller  graduated  circle  to  slip  over  the  pro- 
jecting brass  tube  in  the'  upper  cover.  Tl^is  circle 
should.be  cut  out  of  a  sheet  of  tinned-iron  or  zinc,  and 
the  degrees  of  arc  (of  which  there  should  also  be  360°) 
marked  clearly  upon  it  by  scratching  deeply  with  a 
•sharp  steel  point.  In  the  centre  of  this  circle,  a  hole, 
just  sufficiently  large  to  admit  the  passage  of  the  piece 
"of  brass  tube,  should  be  punched.  This  circle  is  to  be 
slipped  over  the  tube  and  lie  flat  on  the  cover,  but  must 
not  be  fastened  (Town.  A  short  brass  rod,  about  2\  in.  long, 
and  just  thick  enough  to  enter-  freely  into  the  brass  tube, 
is  now  procured,  and  fitted  with  a  circular  brass  head 
made  by  filing  up  a  piece  of  £  in.  sheet  brass  into  \  in. 
in  diameter,  and  soldering  the  rod  in  the  centre  of  the 
circle.  Or,  the  pinion  of  an  old  paraffin  lamp  burner. 


22  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 

with  the  teeth  knocked  off,  may  be  used  instead  of  this 
rod,  provided  it  fits  the  tube.  Whichever  is  used,  a 
short  piece  of  brass  wire  must  be  soldered  at  the  upper 
extremity  of  this  brass  rod,  near  the  head. 

This  index  serves  two  purposes  :  First,  it  prevents  the 
rod  going  too  deeply  into  the  tube ;  secondly,  it  serves 
to  point  out  the  amount  of  torsion,  or  twist,  given  to  the 
wire  or  fibre  which  supports  the  "  stirrup  "  in  which 
is  placed  the  magnetic  needle  or  insulating  rod,  which 
is  used  in  magnetic  or  electric  measurements.  The 
lower  end  of  the  rod  must  project  just  below  the  lower 
end  of  the  tube  which  passes  through  the  cover,  and  to 
this  lower  end  of  the  rod  must  be  soldered,  a  short  length 
of  No.  40  German  silver  v/ire.  To  the  other  end  of  this 
German  silver  wire  must  be  attached,  by  soldering,  a 
"  stirrup,"  in  the  form  of  a  wide  J.  The  length  of  the 
wire,  inclusive  of  the  stirrup,  must  be  such  that  it  just 
.swings  clear  of  the  lower  graduated  circle  when  a  mag- 
netic needle  or  other  rod  is  placed  across  the  stirrup,  and 
the  cover  is  on  the  top  of  the  glass  cylinder.  A  well- 
magnetisea  needle,  a  little  shorter  than  the  diameter  of 
the  cylinder,  completes  the  instrument  if  it  is  to  be  used 
as  a  magnetic  measurer.  The  needle  may  be  a  piece  of 
a  good  steel  knitting  needle,  carefully  magnetised  to 
saturation.  If  required  for  electrical  experiments,  the 
magnetic  needle  must  be  replaced  by  a  light  shellac  rod, 
carrying  at  one  extremity  a  small  disc  of  thin  sheet  brass 
or  copper.  In  this  latter  case  also,  the  channel  in  the 
base  board  must  be  fitted  with  a  bent  brass  wire  shaped 
like  the  letter  L,  furnished  with  a  small  brass  ball  at- 


COULOMB'S  TORSION  BALANCE. 


each  extremity.  This  rod  must  be  carefully  insulated  by 
being  thickly  coated  v/ith  good  red  sealing  wax  to  the 
depth  of  at  least  |  in.  all  round,  except  at  the  two 
extremities  where  the  balls  are^  situate. 

Fig.  5  illustrates  the  different  parts,  and  the  complete 
instrument.  A  is  the  base  board,  with  the  slot  and 
graduated  circle  attached  ;  B  is  the  tin  cover,  with  its 
B 


Fir..  5. 

trass  tube,  the  whole  being  cemented  to  the  top  .of  the 
glass  cylinder  when  once  the  wire  has  been  soldered  to 
the  milled  head  ;  C  is  the  upper  graduated  disc  ;  D  the 
rod  carrying  the  index,  and  actuated  by  the  brass  head » 
to  this  rod  is  attached  the  wire  and  stirrup  E  ;  F  shows^ 
the  instrument  put  together,  and  lettered  homologously. 
§.15."  VOLTA'S  ELECTROPHORUS. — This  is  essentially 
the11  amateur's  electrical  machine.  It  is  at  once  the 
simplest  to  construct,  the  most  efficient  in  action,  and 


24  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEUR'S. 

its  theory  is  most  interesting  as  giving 'the  key  to  the 
continuous-action  electrophori  of  the  present  day.  I 
allude  to  the  Holtz,  to  the  Bertsch,  to  the  Carrd,  and  to 
the  Wimshurst  machine.- 

§  1 6.  To  make  an  electrophorus  will  be  needed  a 
sheet  of  zinc,  from  which  are  cut  two  discs ;  one  being 
,about  2ft.  in  diameter,  the  other  about  4  in.  less.  The 
zinc  should  not  be  more  than  A  in.,  nor  less  than  •sV  in.  in 
thickness.  The  outer  edge  of  bxr  ,  the  discs  must  be 
turned  up  by  careful  hammering*  th  a  mallet,  and  a 
round,  soft,  iron  rod,  a  \  in.  thick,  mus£  be  run  round  the 
edge  of  each  disc,  and  covered  over  with  the  upturned 
edge  of  the  disc.  This  operation  requires  a  considerable 
amount  of  care  and  patience  to  effect  it  neatly.  It  is 
absolutely  essential  to  the  efficiency  of  the  machine  that 
the  edges  should  be  perfectly  round,  without  any  sharp 
angles;  and  this  end  can  only 
be  attained  by  using  a  steel 
tool,  in  shape  something  like 
a  solid  punch,  but  having  a 
semi-circujar  concavity  at  its 
lower  extremity  (see  Fig.' 
6,  A).  By  turning  the  edge 
of  the  zinc  over  the  rod  by 
the  aid  of  this  tool  and  a 
hammer,  a  smooth  border  can 
easily  be  produced.  Should, 
however,  the  amateur  find 
any  difficulty  in  performing  this  operation,  it  would 
be  advisable  for  him  to  have  recourse  to  the  nearest 


FIG.  6  A. 


.  VOLTAS  ELECTROPHORUS.  25 

tinman,  who  will  execute  the  necessary  sleight  of  hand 
for  a  few  pence.  If  well  done  the  two  discs  should 
present  the  appearance  shown  at  Fig.  6,  B  and  C.  It 
will  be  noticed  that  from  the  ce  tre  of  the  smaller  disc 
projects  a  short  piece  of  tube.  This  is  made  of  a  short 
length  of  £  in.  brass  or  zinc  tubing  about  I  in.  in  length, 


FIG.  6  B,  c,  D. 

which  is  soldered  centrally  on  the  smaller  disc.  It  tS 
into  this  tube  that  the  insulating  handle  is  to  be  inserted, 
so  that  the  amateur  may  vary  somewhat  the  diameter 
of  this  tube  to  suit  that  of  his  handle  ;  but  it  should  not 
greatly'exceed  \  in.  in  diameter,  nor  be  less  than  a  \  in. 
Near  one  edge  of  the  smaller  disc  should  be  soldered  a 


a6  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 

short  length  of  \  in.  brass  wire— say,  about  3  in.  in  length 
— bent  into  the  form  of  a  rounded  L  ,  and  bearing  at  its 
extremity  a  brass  ball  about  I  in.  in  diameter.  (Such 
balls  may  be  obtained  from  most  ironmongers  at  about 
sixpence  each.)  If  not  easily  procurable,  it  may  be 
replaced  by  a  leaden  bullet  cast  on  to  the  end  of  the  wire 
(see  Fig.  6,  B).  A  handle  of  some  insulating  material 
must  now  be  provided.  '  In  shop-bought  instruments 
glass  handles  are  generally  seen;  but  these  are  not 
nearly  so  efficient  as  ebonite,  though  very  much  more 
beautiful.  At  most  chemists  ebonite  stirring-rods,  about 
9  in.  long  .and  somewhat  over  a  \  in.  thick,  may  .be 
obtained  for  twopence  or  threepence  each.  One  of 
these  will  serve  our  purpose  admirably ;  and  its  round 
end  should  be  cemented  into  the  central  tube  by  the  aid 
of  a  little  guttapercha  or  Prout's  elastic  glue,  applied  hot 
to  the  end  of  the  rod,  and  the  extremity  of  the  tube 
slightly  pinched  round  it  to  insure  its  not  slipping  out. 
The  larger  disc  must  now  be  filled  with  some  insulation 
composition.  Of  these  there  are  several ;  the  best  is 
perhaps  ebonite;  but  it  is  expensive,  being  about  6s.  6d. 
a  pound ;  however,  it  can  be  obtained  in  very  thin 
sheets,  and  this,  of  course,w  lessens  the  expense.  If 
ebonite  is  used,  a  disc  should  be  cut  that  will  just  fit 
inato  the  larger  zinc  disc,  and  be  fastened  down  to  it 
nicely 'and  smoothly  by  means  of  Prout's  elastic  glue, 
aided  by  equal  pressure  with  a  warm  (not  too  hot)  iron. 
Another  very  good  material  is  gun-paper.  Gun- paper,. 
or  P<*Pyro*yline,  is  paper  which  has  been  immersed  for  a 
few  seconds  in  a  mixture  of  nitric  and  sulphuric  acids, 


VOLTAS  ELECTROPHORUS.  VJ 

and  then  washed  in  an  abundance  of  water.  A  disc  of 
this  papyroxyline,  cut  to  reach  quite  to  the  bead  of  the 
larger  zinc  disc,  may  be  cemented  to  the  said  disc  by 
means. of  any  spirit  varnish  (say,  white  hard)  and  when 
quite  dry>  the  surface  also  varnished  with  the  same,  to 
prevent  the  paper  absorbing  moisture,  which  it  would 
otherwise  be  apt  to  do.  If  papyroxyline  is  used,  it 
should  be  made  from  stout  millboard  of  at  least  ^in. 
thick,  and  \  will  be  better  still.  This  material  is  prac- 
tically inde'structible,  and  does  not  deteriorate,  as  does 
the  ebonite,  by  the  oxidation  of  the  surface,  or  as  the 
usual  shellac  composition,  by  splitting  up  from  the 
zinc. 

The  last  and  most  usual  composition  for  the  larger 
disc  is  a  mixture  of  Venice  turpentine,  wax,  and  shellac. 
To  make  it  of  even  consistency,  it  is  necessary  to  pro- 
ceed as.  follows  : — ^ 

Take  best  shellac .3  parts. 

^fellow  beeswax 2  parts. 

Venice  turpentine  ........................  I  part. 

Place  the  shellac  in  an  earthen  pipkin,  and  stand  it  over 
a  gentle  fire.  Watch  until  melted.  When  melted  add 
the  beeswax.  When  well  mixed  pour  in  the  Venice 
turpentine.  Again  stir,  and  when  mixed,  pour  into  the 
large  zinc  disc,  which  must  have  previcusly  been  warmed 
to  nearly  the  same  temperature  and  placed  on  a 
perfectly  level  table.  Should  any  difficulty  be  experienced 
in  getting  the  mixture  to  flow  evenly  over  the  disc,  it 
may  bs  accisted  by  ironing  over  with  a  tolerably  hot 
iron.  Should  this  mixture  be  preferred  to  the  ebonite, 


28  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 

or  papyroxyline  disc,  then  care  must  be  taken  to  have  a 
sufficiency  to  fill  the  zinc  disc  up  level  with  the  wfre 
beading,  because  if  the  coating  of  resinous  mixture  be 
left  too  th'in,  it  will  all  split  away  on  being  beaten  during 
excitation.  Fig.  6,  D,  shows  the  electropEorus  in  its 
finished  form,  and  in  position  ready  for  use. 

To  put  in  action,  it  is  only  necessary  to  Remove  the 
covering  disc  a\  by  its  handle  b',  and  to  excite  the 
resinoys  cake  (ebonite  or  gun-paper)  by  beating  it  with 
about 'half  a  yard  of  warm,  dry  flannel,  or  better  still,  a 
cat's  skin.  Then  if  the  cover  be  placed  on  the  centre 
of  the  cake,  the  tipper  disc  touched  with  the  finger,  and 
then  rapidly  raised  to  a  height  of  6  or  8  in.  by  means  of 
the  insulating  Candle  b',  care  being  taken  not  to  let  any 
part-  of  the  clothes  or,  body  touch  the  disc,  a  fine  2  in. 
spark  may  be  drawn  from  the  knob  by  approaching  the 
knuckle  or  another  brass  ball  to  it  rapidly.  Sparks  may 
be  obtained  almost  indefinitely  by  again  lowering  the 
dis'cs,  touching,  and  again  raising.  I  strongly  recom- 
mend every  student  to  make  an  electrophorus,  and  not 
to  rest  satisfied  until  he  has  Jully  mastered  the  theory 
of  its  action.  When  he  understands  this,  he  will  have 
got  pretty  deeply  into  the  theory  of  induction,  and  will 
be  prepared  to  grasp  the  theory  ;of  the  Wimshurst 
machine — a  machine  which  will  probably  play  a  very 
important  part  in  future  applications  of  electricity  to 
the  arts. 

§  17.  BERT'SCH'S  MACHINE,— Though  this  is  not  the 
best  form  of  continuous-action  electrophorus,  yet  as  its 
construction  is  extremely  simple,  and  leads  to  a  thorough 


MOUNTING  A  GLASS  PLATE.  s$ 

comprehension  of  the  principles  on  which  the  more 
efficient  forms  are  dependent,  it  will  be  advisable  for 
the  student  to  undertake  it.  The  Bertsch  machine  may 
be  made  with  the  rotating  plate  either  of  glass  or  ebonite. 
Both  forms  will  be  described,  beginning  with  the  glass 
plate  form,  as  the  mode  of  mounting  a  .glass  plate  on  a 
spindle  is  generally  regarded  as  a  "  poser  "  by  amateurs. 

§  1 8.  MOUNTING  A  GLASS  PLATE. — There  are  two 
modes  open  to  the  amateur — viz.,  by  drilling  a  hole 
through  the  glass  of  sufficient  size  to  take  the  spindle, 
and  screwing  or  cementing  two  cheeks  against  the  glass 
plate  ;  or  by  cementing  two  cheeks,  one  on  either  side  of 
the  glass  plate,  exactly  central,  and  opposite  one  another, 
these  cheeks  having  the.  spindle  (of  which  there  are  two 
halves)  inserted  at  their  centres. 

§  19.  To  drill  a  glass  plate  requires  more  patience 
than  skill,  though  both  are  needful.  In  the  first  place 
the  glass  must  be  cut  to  an  exact  circle,  and  its  centre 
marked.^ 

To  do  this  the  amateur  should  procure  a  stout  piece 
of  brown  paper,  and  with  the  compasses  strike  out  a 
circle  of  the  size  of  which  he  intends  his  glass  plate  to 
be.  (A  very  convenient  size  for  most  electrical  machines 
is  18  in.  in  diameter.) 

Keeping  this  marked  paper  as  a  template,  our  student 
cuts  out  a  similar  circle  in  rather  stout  millboard.  He 
then  places  his  glass  plate  upon  a  perfectly  flat  table, 
with  the  millboard  disc  over  it.  A  small  dab  of  hot 
Prout's  cement  applied  to  the  glass,  will  hold  the  mill- 
board to  the  glass  so  that  it  shall  not  move  during  the 


30  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 

cutting  of  the  circle.  The  operator  now  traces  with  a 
diamond  or  glass-cutter  a  circle  all  round  the  millboard 
guide,  being  careful  not  to  press  so  heavily  as  to  split 
the  glass,  or  so  lightly  as  not  to  cut  To  a  practised  ear 
the  peculiar  whistle  tells  when  the  glass  is  being  cut, 
and  when  only  scratched.  The  circle  being  thus  traced 
on  the  glass  with/ the  diamond,  the  millboard  guide  is 
removed,  and  the  corners  of  the  glass  cut  so  as  to 
permit  the  curved  pieces  being  pulled  asunder.  The 
four  diagonal  cuts  must,  of  course,  come  quite  close  to 
the  periphery  of  the  circle  cut  on  the  glass.  A  cursory 
examination  of  Fig.  7  will  render  this  clear.  A  repre- 


FIG.  7. 

sents  the  circle  first  traced  on  the  glass  ;  B  the  four 
diagonal  cuts,  which  enables  the  quadrantal  segments  C 
to  be  detached.  To  insure  the  glass  breaking  where  it 
is  cut,  and  nowhere  else, a  little  care  is  necessary.. 

Firstly,  the  operator  must  see  -that  the  glass  is  really 
cut,  and  not  simply  scratched.  To  know  this,  when  he 
has  traced  his  line  or  curve  with  the  diamond  or  cutter, 
he  must  turn  the  glass  ove.r  and  examine  the  line  on 
the  other  side.  If  it  looks  like  a  mere  white  line,  not 
penetrating  below  the  surface,  it  is  but  a  scratch.  If, 
on  the  contrary,  it  shows  as  a  glistening  crack  right 
through,  it  is  a  true  cut.  By  gently  tapping  with  the 


MOUNTING  A  GLASS  PLATE.  3» 

cutter  on  the  wrong  side  of  the  glass  a  scratch  may 
frequently  be  converted  into  a  ait,  but  it  is  necessary  to 
raise  the  glass  from  the  table  "at  one  extremity  while 
tapping,  and  to  tap  on  the  spot  only  where  it  is  required 
to  decide  the  fracture.  When  the  line  shows  as  a 
glistening  fracture  in  its  entire  length,  then,  by  holding 
the  glass  plate  between  the  finger  and  thumb  of  the  left 
hand,  with  the  "cut  'side  uppermost,  and  the  cut  close  to 
the  thumb,  the  operator  will  find  no  difficulty  in  detach- 
ing the  diagonals,^  and  afterwards  the  quadrantal  seg- 
ments, by  a  steady  downward  pressure  of  the  right 
hand,  applied  to  the  other  side  *of  the  cut  line.  Should 
any  pieces  refuse  to  Jareak* quite  truly,  they  may  be 
"  nibbled "  toff  Uy  means  of .,  the  wards  of  a  key,  or  the 
slots  in  the  glass-cutter.  When  the  circle  has  been  cut 
to'satisfaction,  the  edges  should  be  smoothed  by  grind- 
ing ;  but  as  this  is  best  done  when  the  disc  is  mounted 
on  its  spindle,  the  directions  as  to  the  mode  of  grinding 
are  deferred  until  the  method  of  mounting  has  been 
described.  The  glass  disc  must  now  be  placed  on  drill- 
ing-table, constructed  specially  for  this  purpose.  It  con- 
sists, as  shown  in  Fig.  8,  in  a  flat  board,  A,  about  I  in. 
thick,  20  in.  long,.£nd  about  I2in.  wide.  From  each 
extremity  of  this  board  rises  a  standard  B  B'  about  3  in. 
wide  by  £  in.  thick,  and  10  in.  or  12  in.  high.  There  are 
two  cross  pieces  C  C',  joining  these  upright  standards 
together,  one  at  the  top,  and  one  withiri  an  inch  of  the 
bottom.  These  cross  pieces  have  each  a  circular  hole 
perforated  through  the  centre,  of  a  trifle  over  the 
diameter  of  the  desired  hole  in  the*  glass  (generally  $  in. 


'32  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 

in  diameter).  Through  these  holes  passes  a  copper  tube, 
D,  \  in.  in  diameter  and  14  in.  long.  Around  the  upper 
extremity  of  this  copper  tube  is  cast  a  "aeavy  flange  of 
lead,  E,  weighing,  say,  at  least  2  Ib.  Before  using  this  tool, 
the  lower  extremity  of  the  tube  must  be  "  upset " — id. 
estt  made  irregular — and  broadened  a  little  by  hammer- 
ing on  its  edges.  This  must  be  done  to  prevent*  the 
glass  being  split.  To  work  this  drill,  the  glass  being 
fastened  in  its  place  by  means  of  three  corks  and  screws 
with  its  marked  centre  just  under  the  centre  of  the 


FIG.  8. 

copper}  tube,  a  small  quantity  of  ordinary  emery  ig 
poured  into  tne  top  of  the  tube,  to  which  are  added  a  feV 
drops  of  oil  of  turpentin6  as  a  lubricator.  A  gut-band, 
F,  is  now  passed  once  around  the  tube,  and  fastened  at 
each  end  to  a  tolerably  stiff  bow,  G.  Moving  the  bow 
backwards  and  forwards  communicates  a  reciprocating 
rotary  motion  to  the  drill,  and  this,  aided  by  plenty  of 
patience  _  and  a  little  emery  and  turpentine,  will  cut  a 
clean  round  hole  in  the  glass.  This  having  been 
effected,  the  next  step  .  is  to  cement  the  spindle  in  its 


MOUNTING  A  GLASS  PLATE.  33 

place.  The  spindle  may  be  from  £  in.  to  £  in.  in  diameter 
and  should  be- furnished  with  wooden  cheeks  or  washers 
(consisting  of  cotton  reels  cut  in  half),  which  fit  them 
pretty  tightly.  One  of  the  cheeks  should  be  glued  to 
the  spindle  (nearly  at-  its  centre)  with  Kay's  coaguline, 
or  similar  acetic  glue.*  "  When  quite  dry  and  set  firm' 
the  surface  of  the  cheek- should  be ' painted  over  with 
"bicycle  tire  cement,"  and  the  glass  plate  immediately 
slipped,  over  the  spindle  into  its  place.  *  The  other  cheelc 
should  now  be  treated  m^the  same  manner;  that  is  to 


Fio.,9. 

say,  treatecTwith  " bicycle  cement  ".on  its  face,  and  the 
portion  'of  the  spindle  near  the  glass  plate  -thickly 
brushed  over  with  acetic  glue.  The  upper  cheek  must 
now  be~slipped^on  to  the  spindle  and  pressed  tightly 
down  upon  the  glass,  the  whole  being  left  undisturbed 
for  some  hours  until  quite  set  and  firm.  Fig.  9  will, 

*  This  useful  cement  is  made  by  soaking  good  glue  in  cold  water  until! 
quite  soft,  pouring  away  the  water  and    adding  sufficient  glacial  acetic 
acid  to  cover  the    glue.      A    slight   heat  will    render  the  Whole  liquid, 
when  .it  should  be  poured  into  a  bottle  kept  corked  for  use. 
D 


34   ELECTRICAL  INSTR UMENT MAKING  FOR  AMA  TEURS.\ 

illustrate  the  appearance  of  the  disc'  when  the  spindle, 
and  lower  cheek  have  been  affixed.  A  is  the  glass  plate, 
B  the  lower  spindle,  C  the  lower  cheek,  cemented,  D  the 
upper  cheek,  ready  to  be  pressed  down.  The  length  of 
the  spindle  and  the  nature  of  its  attachments  will  depend 
on  the  use  to  which  the  plate  will  hereafter  be  put.  In 
the  present  instance,  the  glass  plate  being  18  in.  diameter, 
the  spindle  should  be  (a  steel  rod)  at  least  i$in.  long, 
and  should  have  a  shoulder  turned  down  at  each  end  to 
the  length  of  about  i  in. 

When  the  spindle  and  glass  plate  are  thoroughly 
cemented  together,  a  small  wooden  pulley  about  i£  in. 
diameter,  similar  to  those  used  for  roller  blinds,  must  be 
cemented  close  to  one  extremity  of  the  spindle,  just 
before  the  shoulder.  This  serves  to  take  the  band  or 
gut,  by  means  of  which  motion  is  dommunicated  to  the 
glass"  disc. 

§  20.  MOUNTING  A  GLASS  PLATE  WITHOUT  DRILLING. 
This  method  is  much  simpler  than  the  former,  and  is 
in  motet  cases  preferable.  It  is  certainly  superior,  in  the 
case  of  such  machines  as  the  Bertsch,  the  Carr<5,  the 
Holtz,  the  Wimshurst,  and,  indeed,  all  "  induction  " 
machines  ;  and  it  is  quite  equal  for  frictional  machines, 
except  when  the  plates  are  very  large,  say,  over  2  ft.  in 
diameter.  To  mount  a  plate  in  this  manner  all  ivhat  is 
necessary  is  to  place  the  plate  on  the  marked  paper 
which  has  been  kept  as  a  template  (§19),  so  as  to  be 
able  to  find  the  exact  centre.  Over  this  must  be  pasted 
(with  bichromated  paste)  circlets  of  brown  paper,  one 
on  each  side  of  the  glass,  of  the  same  size  as  the 


MOUNTING  A  GLASS  PLATE  WITHOUT  DRILLING.     3$ 

wooden  cheeks  (made,  as  beforesaid,  from  a  cotton  reel 
sawn  in  half).  The  bichromated  paste  is  made  as 
follows  : — Flour,  two  teaspoonfuls  ;  water,  four  ounces  ; 
bichromate  of  potash,  five  grains.  The  flour  must  be 
rubbed  up  to  a  smooth  batter  with  the  water,  then  placed 
in  a  small  saucepan  over  a  source  of  heat,  and  kept  stirred 
until  it  "boils.  The  bichromate  of  potash,  in  powder, 
is  placed  in  a  jam  pot,  and  the  boiling  paste  poured! 
upon  it,  with  constant  stirring.  This  paste  must  be 
kept  in  the  dark.  The  brown  paper  used  foi  the 
circlets  should  be  of  a  good  stiff  fibrous  texture,  such  as 
is  used  for  packing  heavy  goods,  and  should  be  well 
soaked  in  the  paste,  previous  to  placing  on  the  glass 
disc.  When  the  circlets  have  been  fastened  on,  the 
glass  disc,  with  its  paper  circlets,  must  be  exposed  to 
good  sunlight  for  an  hour  or  two.  This  sunning  sets  up 
a  chemical  change  in  the  bichromate,  and  renders  the' 
paste  insoluble,  so  that  it  does  not  easily  detach  from 
the  glass  plate.  When  quite  dry,  the  wooden  cheeks 
(with  the  spindles)  are  to  be  glued  to  the  paper  circlets, 
and  the  glue  must  contain  a  few  grains  of  bichromate  of 
potash,  so  as  to  insure  that  it  shall  not  be  affected  by 
damp. 

§  21.  As  there  is  no  hole  through  the  glass  disc,  the 
amateur  may  find  it  rather  difficult  to  get  the  spind!e 
(which  is  now  in  two  halves)  to  be  exactly  in  a  straight 
line.  The  two  halves  are  shown  at  Fig.  10,  where  a  <t 
are  the  two  half  reels,  to  which  have  been  attacjhed  with 
acetic  glue  two  steel  rods,  b  b',  about  7^  in.  long.  To 
ensure  these  being  perfectly  opposite  one  another  on 


3b  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 

the  glass  disc,  it  is  only  necessary  to  glue  one  on,  and 
let  it  dry  perfectly;  then,  having  placed  the  disc  with 
the  spindle  downwards  on  a  block  of  wood,  in  which  a 
hole  has  been  drilled  of  the  same  diameter  as  the 


spindle  (as  shown  at  Fig.  10,  <:),  to  glue  the  other  on  as 
nearly  central  as  one  can  judge  by  eye.  On  causing 
the  disc  to  rotate  on  the  lower  spindle  as  on  a  pivot,  it 
is  easily  seen  whether  the  upper  spindle  is  in  a  line 


THE  BER7SCH  MACHINE.  37 

with.it  or  not,'  as  it  will  in  this  .lattef  case  describe 
a  circle '  instead '  of  remaining  apparently  motionless. 
While  the  glue  is  still  warm,  any  necessary  correction  in 
position  can  be  made.  Both  spindles  being  now  glued 
on,  the  disc  should  be  allowed  to  dry — if  possible — in  the 
sunshine  ;  and  then  the  wooden  cheeks  neatly  varnished 
(especially  round  the  junction  between  the  glass  and 
the  woo'd)  with  a  varnish  made  by  mixing  good  white 
hard  varnish  with  Chinese  red. 

§22.  The  disc  being  now  mounted  in  one  "or  oll^r 
mode,  we  can  proceed  to  fix  it  in  the  standards'  on 
which  it  is  intended  to  rotate.  To  this  end,  a  base 
board,  about  18  in.  long  by  12  in.  wide,  and  about  I  in. 
thick,  is  planed  -up,  and  two  uprights,  about  16  in.  in 
height  and  I  in.  square,  let  into  the  sides  and  screwed 
thereto,  as  shown  at  Fig.  10,  d  d.  With  a  rat-tail  file, 
two  semi-circular  •  grooves  are  cut  in  the  top  of  these 
standards,- exactly  opposite  one  another,  in  which  thet 
spindles  of  the  disc  must  run  easily.  The  glass  disc 
having  been  placed  with,  its  spindle  resting  in  these 
grooves  as  bearings, '  two  caps  are  fashioned  out  of 
square  pieces  of  wood  to  fit  over  the  upright  standards, 
and  are  screwed  in  their  places  by  means  of  two  screws 
The  use  of  these,  caps  (of  which  one  is  figured  at  e)  is  to 
prevent  the  spindle  rising  out  of  the  groove  during 
rotation.  The  standards  d  and  d'  must  be  such  a  dis- 
tance apart  that  the  .ends  of  the  spindle  that  have  been 
turned  down  (§19,  last  paragraph  but  one)  rest  in  the 
grooves  just  up  to  the  shoulder. :" 

§  23.  It  will  be  remembered  that  it  was  recommended 


38  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 

to  attach  a  small  wooden  pulley  to  one  of  the  steel  rods 
(§  19).  It  will  now  be  necessary  to  place  a  driving- 
wheel  below  this,  which  can  be  turned  by  means  of  a 
projecting  handle,  and  communicate  its  motion  to  the 
small  pulley  by  the  aid  of  a  gut.  This  driving-wheel 
should  be  of  wood,  about  6  in.  in  diameter,  \  in. 
thick,  and  have  a  groove  cut  in  its  circumference 
about  -J  in.  deep.^  If  our  amateur  has  a  lathe,  this  is 
an  easy  matter  ;  if  not,  he  can  cut  the  wheel  out  of  a 
l-irch  deal  board  with  a  handsaw,  having  first  struck 
a  6  in.  circle  on  it  with  the  compasses,  as  a  guide  in 
sawing  it.  The  bevel  in  the  edge  can  be  cut  with  a 
good  triangular  file.  This  must  be  keyed  to  an  iron 
rod,  about  \  in.  thick  by  4  in.  long,  as  shown  at  ft 
Fig.  10,  where  it  will  be  observed  that  the  rod  or 
spindle  projects  more  at  the  one  extremity  than  at  the 
other.  The  longer  extremity  passes  through  a  hole 
in  the  standard  d1,  made  \at  such  a  height  as  to  allow 
this  driving-wheel  just  to  clear  the  base  board.  A 
third  standard,  of  the  shape  of  a  letter  J_>  is  cut  out  ol 
J-inch  stuff  planed  up,  reaching,  when  the  head  of  the 
±  is  on  the  base  board,  to  the  same  height  as  the  other 
two,  g,  Fig.  IO.  A  hole  is  drilled  through  this  third 
standard  to  admit  of  the  passage  of  the  short  end  ol 
the  driving-wheel  spindle  ;  and  a  channel  (similar  to 
those  in  the  other  two  standards)  is  filed  in  the  top. 
The  driving-wheel  having  been  put  in  its  place,  this 
third  standard  is  placed  against  it,  leaving  sufficient 
room  for  it  to  rotate  freely.  The  standard  is  then 
fastened  down  to  the  base  board  by  means  of  a  screw 


THE  BERTSCH  MACHINE.  39 

in  each  branch  of  the  J.-  The  glass  plate  is  now  put 
in  its  place,  the  spindle  resting  in  the  grooves  on 'the 
tops  of  the  standards.  The  caps  e  are  then  screwed  on.  A 
gut  band  is  attached  so  as  to  pass  round  the  driving- whee?. 
and  the  pulley  fastened  to  the  spindle  of  the  qlass  disc. 
A  small  metal,  or,  better,  wooden  handle  is  screwed  to 
the  projecting  spindle  of  the  driving-wheel  (Fig.  11,  H). 


FIG.  n. 

§  24.  A  piece  of  sheet  ebonite  about  7  in.  long  by 
4  in.  wide  is  cut  rounded  at  the  top,  and  fastened  at 
the  bottom  fo.  one  side  of  a  little  wooden  stand,  which 
can  easily  be  effected  by  the  aid  of  a  couple  of  flat- 
headed  screws,  the  heads  of  w'.iich  must  be  carefully 
covered  over  with  Prciut's  clastic  glue  (Fig.  n,  v). 


40  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 

This  littje  piece  of  ebonite  (technically  known  as  the 
"sector'*)  and  its  stand  must  be  attached  to  the  base 
board,  parallel  with  and  close  to  the  glass  disc,  without 
actually  touching  it  As  it  is  necessary  to  remove  it 
for  the  purpose  of  excitation,  it  must  not  be  per- 
manently fastened  to  the  base  board,  but  only  held 
there  by  means  of  a  small  thumb-screw.  Opposite  this 
sector,  but  on  the  other  side  of  the  glass  plate,  is  a 
"  comb,"  made  by  soldering  half  a  dozen  brass  brads 
to  a  stout  brass  rod  about  4  in.  long.  A  precisely 
similar  cornb  faces  the  plate  at  its  upper  diameter 
(see  Fig.  1 1,  c'  and  C).  These  combs  are  soldered  by 
their  middles  to  stout  brass  rods  R  R'.  The  lower  one 
passes  through  the  top  of  the  ebonite  rod  E,  which  must 
be  solid,  and  forms  a  prolongation  of  the  cap  of  the 
standard  s'.  The  lower  rod  R'  terminates  in  a  bras? 
ball  B  :  the  upper  one  in  a  short  piece  of  split  brass 
tube  D,  through  which  passes  with  some  friction  another 
•brass  red  r,  to  'which  is  affixed  at  one  end  an  ebonite 
handle  M,  and  at  the  other  a  second  brass  ball  B'.  The 
split  tube  D  allows  this  rod  r  to  be  approached  to  or 
withdrawn  from  the  ball  B.  It  is  usual,  in  order  to 
increase  the  effects,  to  hang  a  Leyden  jar  (or  similar 
condenser)  between  R  and  R',  the  inner  coating  being 
in  contact  with  the  one,  and  the  outer  with  the  other, 
of  these  rods.  Contact  with  earth  can  also  be  made  at 
will  by  hanging  a  chain  to  the  rod  near  the  ball  B,  and 
allowing  it  to  touch  the  table,  D'. 

§   25.    GARRETS    DIELECTRJCAL    MACHINE.— The 
principle  of  this  machine  is  precisely  similar  to  that  of 


CARK&S  DIELECTRICAL  MACHINE.-  41 

the  Bertsch,  but  it  possesses  the  great  advantage  of 
being  less  affected  by  the  state  of  the  atmosphere.1 
This  is  owing  to  the  employment  of  a  continuously 
excited  plate  of  glass  t>r  ebonite,  instead  of  the  "sector  " 
(§  24)  used  in  the  Bertsch  machine.  With  two  plates, 
respectively  15  in.  and  19  in.  in  diameter,  sparks  from 
6  in.  to  7  in.  in  length  may  be  obtained  without  a  con- 
denser'; and  if  a  pair  of  Leyden  jars  be  added,  as  in 
most  modern  induction  machines,  this  length  may_- be 
considerably  exceeded. 

§  26.  With  the  knowledge  acquired  in  mounting  glass 
plates  on  spindles  (§  20)  the  amateur  will  find  no  difficulty 
in  constructing  an  effective  Carr^  machine.  The  plates 
may  be  either  both  ebonite,  or  both  glass  ;  or  one  ebonite 
and  the  other  glass.  Personally,  I  prefer  both  glass,  pro- 
vided the  glass  be  of  the  required  non-conducting  quality. 
Glass  varies  very  much  in  its  insulating  power,  and  this 
can  only  be  ascertained  by  actual  trial.  Fortunately,' 
this  is  not  a  difficult  matter.  It  is  only  needful  to  dry 
the  sheet  of  glass  cautiously  before  a  moderate  fire  ;  rub 
it  briskly  with  a  silk  handkerchief  (previously  dried)  so 
as  to  electrify  the  surface.  If  with  an  electrophorus 
shield  of  about  12  in.  in  diameter  (§  16),  a  spark  of  about 
2  in.  in  length  can  be  obtained,  the  glass  may  be  con- 
sidered sufficiently  insulating  for  our  purpose;  if  much 
less,  it  had  bettei  be  rejected.  The  suitable  kind  of  glass 
havjng  been  selected,  it  is  cut,  precisely  as  previously! 
directed  (§  19),  into  two  discs,  one  15  in.,  the  other  loin.i 
in  diameter.  These  two  discs  should  be  mounted  in: 
either  of  the  modes  previously  Described  (§§  19,  20)  ow 


43 '\  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 

spindles,  projecting  6  in.  on  each  side  of  the  discs, 
These  spindles  should  be  made  of  J- in.  circular  iron  rod, 
inserted  into  the  half-reels  ;  and  afterwards  the  rods 
should  be  carefully  covered  with  a  casing  of  thick  brov/n 
paper,  which  has  previously  been  soaked'  in  melted 
paraffin  wax.  This  casing  of  paraffin  paper  must  bs 
neatly  glued  roun4  the  rods,  so  as  to  form  a  smooth 
surface,  and  when  the  whole  is  dry  should  receive  a 
coating  of  the  red  varnish  mentioned  at  §  10.  The  next 
step  is  to  cut  out  the  base  board  from  a  piece  of  inch 
stuff,  which  may  be  either  deal  or  mahogany.  In  either 
case,  as  the  size  must  be  II  in.  by  15  in.,  good,  well* 
seasoned  wood  must  be  used,  otherwise  warping  will 
take  place,  and  'spoil  the  whole  instrument.  In  the 
centre  of  the  two  long  sides  of  this  base  board  are 
mortised  two  standards.  Both  these  standards  should 
be  cut  from  planed  stuff,  2  in.  wide  by  l\  in.  thick — one 
(a)  should  be  about  16  in.  in  length,  the  other  (b)  about 
B  in.  Previous  to  being  glued  into  the  base  board,  the 
necessary  bearings,  &c,  to  take  the  ends  of  the  spindles, 
should..be  made  in  them. 

§27.  As  it  is  very  convenient  to  be  able  to  remove 
and  replace  the  glass  plates  at  will,  it  is  advisable  to 
make  these  bearings  in  the  following  manner : — Having 
placed  the  standards  in  their  sockets  temporarily,  the 
smaller  glass  disc  is  held  with  its  spindle  resting  against 
the  two  standards  at  such  a  height  from  the  base  board 
that  the"  edge  of  the  disc  clears  it  by  about  I  in.  A 
mark  is  made  on  both  standards  at  the  point  where  tile 
spindles  touch  the  standards.  An  assistant  now  holds 


•CARRES  D1ELECTRICAL  MACHINE. 


43 


fhe  smaller  disc  in  this  position,  while  the  operator  takes 
fhe  larger  glass  disc,  and  holds  its  spindle  against  the 
higher  standard  at  such  a  height  that  the  edge  of  this 
clears  the  edge  of  the  reel  of  the  lower  disc  by  about 
J- jn.  A  glance  at  Fig.  12  will  make  this  clear.  The 
position Jitjwhich  the  two  discs  must  rest  on  the  stan- 


FIGS.  12  &  13. 

dards  to  ride  clear  of  the  base  board  and  of  each  other 
having  thus  been  found  and  marked,  the  standards  are 
removed,  and  a  slot  cut  into  them  a  little  higher  than 
the  first  line,  inclining  a  trifle  downwards,  and  not  ex- 
ceeding in  depth  (when  finished)  the  thickness  of  the 
'iron  spindle.  Fig.  13  shows  the  position  of/  the  two 


44  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 

slots  in  the  standards  at  a'  and  b' ;  while  c  shows  a  third 
slot  in  the  centre  of  the  top  of  the  higher  standard,  and 
reaching  to  the  top  line,  in  which  the  spindle  of  the 
larger  disc  can  ride.  These  slots  must  all  be  rounded 
(where  the  spindles  rest)  with  a  rat-tail  file.  Square 
pieces  of  wood,  about  J  in.  thick,  are  made  to  fit  over 
these  slots,  so  as  to  keep  the  spindles  from  rising  out  of 
their  slots  during  rotation  ;  and  these  squares  are  held 
in  their  places  by  screws,  as  shown  at  d  dt  Fig.  13.  At 
the  top  of  the  shorter  standard  (b}  must  now  be  cut, 
with  a  centre  bit,  a  circular  hole,  about  three-quarters 
of  an  inch  in  diameter,  and  reaching  nearly,  but  not 
quite,  to  the  lateral  slot  in  depth.  Into  this  hole  is 
cemented,  with  marine  glue,  a  solid  ebonite  rod  f  in. 
diameter,  and  about  17  in.  length.  This  rod  serves  to 
insulate  the  upper  and  lower  qombs  ;  and  also  as  a 
support  for  the  spindle  of  the  large  disc.  This  rod  is 
shown  at  Fig.  14,  e.  At  its  lower  half  is  a  stout  brass 
ring  e',  which  bears  on  one  side  the  brass  rod,  co  which 
is  attached  the  lower  comb_/",  and  on  the  other  a  brass 
rod  carrying  a  ball  g,  about  I  in.  in  diameter,  also  in 
brass.  Three  holes  are  bored  in  the  sides  of  this  ring, 
two  diametrically  opposite  to  one  another,  which  mu.st 
be  tapped  and  screwed  to  take  the  rods ;  and  a  third 
also  tapped  anJ  screwed  to  take  a  small  set-screw,  to 
hold  the  ring  and  its  comb,  &c.,  at  the  desired  height. 
About  half-way  up  the  ebonite  rod,  exactly  opposite 
the  highest  slot  in  the  tallest  standard,  is  a  wooden 
collar  ht  preferably  of  mahogany  or  other  hard  wood. 
This  is  cemented  to  the  ebonite  rod,  and  has  a  small 
orifice  in  its  centre  (facing  the  highest  standard),  in 


CARRES  D1ELECTRICAL  MACHINE. 


45 


-which  the  end  of  the  spindle  of  the  longer  disc  can 
enter. 

A  brass  ball  z,  caps  the  ebonite  rod,  and  this  ball  is 
provided  with  a  metal  comb  /,  and  varnished  paper  comb 
f>  at  one  extremity,  and  a  ball  through  which  passes 
with  friction  a  metal  rod  and  knob  k  at  the  other. 
This  latter  rod  has  an  insulating  handle  /,  by  means  of 


FIG.  14. 

which  the  knob  k~  can  be  approached  to,  or  removed 
from,  the  ball  g  at  will.  These  balls  may  either  be  in 
brass,  or  in  lead.  In  the  latter  case  the  amateur  may 
cast  them  on  the  brass  rods,  and  drill  a*  hole  through 
the  upper  one  for  the  sliding  rod  k\  to  pass  through.  A 
piece  of  wash-leather  glued  on  one  side  of  the  hole  with 
.acetic  glue  will  give  sufficient  friction  to  hold  the  sliding 


46  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 

rod  in  any  position.  If  a  condenser  be  used,  it  should 
take  the  form  of  a  double  Leyden  jar,  and  can  easily  be 
made  by  coating  the  inside  of  two  glass  pomatum, 
bottles,  about  3  in.  high,  with  tinfoil,  to  within  i  in.  of 
the  top.  The  tinfoil  should  be  cut  to  the  height  (about 
2  in.X  then-pasted  on  one  side,  and  finally  slipped  into 
the  bottle  and  pressed  into  its  place.  The  bottles  are 
covered  in  like  manner,  and  to  the  same  height  with 
tinfoil  on  their  outsides,  rare  being  taken  to  paste  tinfoil 
on  the  bottom  as  well  as  the  side.  When  quite  dry  the 
two  bottles  should  be  attached  To  one  another  by  their 
bottoms  by  means  of  a  roll  of  brown  paper  glued  round 
both  together  ;  but  not  reaching  beyond  the  tinfoil. 
When  dry,  the  jars  may  be  varnished  all  over  outside 
with  red  varnish.  A  cork  is  then  fitted  to  each,  and 
through  these  corks  are  forced  wires,  one  end  of  which 
touches  the  tinfoil  at  the  bottom  of  the  jars,  the  other 
end  terminafmg  in  hooks,  by  means  of  which  the  double 
jar  can  be  hung  on  the  upper  rod  /,  and  touch  the  lower 
one  g.  Two  pieces  of  wood,  about  5  in.  in  height  and  of 
the  shape  shown  at  n  ri,  are  cut  out  of  £  in.  stuff,  and 
the  inner  sides  of  these  are  covered  with  a  wash-leather 
cushion,  stuffed  with  horsehair  and  tinfoil.  These 
cushions,  which  serve  to  excite  the  lower  disc,  are  fas- 
tened to  the  base  board  by  two  screws,  o  d.  Bosses, 
made  of  reels  sawn  in  halves,  are  geared  to  the  spindles 
p  p'  p" .  The  covering  on  the  spindle  (§  26)  must  be 
removed  at  the  points  where  they  ride  on  the  standards. 
§  28.  To  give  the  requisite  motion  to  the  two  discs,  a 
wooden  pulley  of  about  6  in.  in  diameter,  and  £  in.  thick, 


HOLTZ  STAZBINE.  47 

having  a  handle  r,  projecting  from  it,  is  keyed  or 
screwed  to  the  lower  spindle  ;  and  a  small  pulley,  about 
I  in,  in  diameter,  is  in  like  manner  keyed  to  the  upper 
spindle.  A  crossed  gut-band,  shown  at  a,  completes 
the  driving  apparatus.  This  band  must  be  crossed,  as 
the  plates  must  rotate  in  opposite  directions. 

§  29.  HOLTZ  MACHINE.— Although  this  machine  is 
far  outdone  in  point  of  efficiency  by  the  Voss  and  the 
Wimshurst  form,  yet,  as  it  is  historically  interesting  and 
presents  no  particular  difficulty  in  construction,  a  simple 
form  will  be  described  in  these  pages. 

The  first  thing  to  be  prepared  is  a  good  sound  base 
board,  about  16  in.  long,  by  1 1  in.  wide,  and  i£  in.  thick. 
It  is  not  material  whether  this  be  of  pine  or  mahogany, 
but  it  is  essential  that  it  should  be  thoroughly  well  sea- 
soned. This  base  board,  with  the  position  of  the  mortise 
holes  to  be  cut  in  it,  to  admit  the  standard  and  glass 
pillars,  is  shown  at  Fig.  1 5,  where  A  is  the  hole  in  which 
is  inserted  the  main  wooden  standard  that  supports  the 
rotating  disc.  B  and  B'  are  two  smaller  apertures  to| 
receive  the  two  shorter  standards  (also  of  wood)  between! 
which  runs  the  driving  pulley.  C  and  c'  are  the  holes 
into  which  are  cemented  the  two  glass  pillars  that  serve 
to  carry  the  combs  and  conductor  rods.  Two  small 
cleats  are  giued  on  at  d  and  </,  to  hold  the  stationary 
"disc"  in  position;  but  these  had  better  be  left  until 
after  the  "  disc  "  is  in  position.  I  say  "  disc  "  because 
in  the  older  form  of  machine,  the  stationary  plate  really 
vas  a  disc,  but  in  this  machine  it  will  take  the  form  of  a 
square  plate  ^6in.  by  i6"in. 


48  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 

§  30.  The  next  operation  consists  in  preparing  the 
wooden  standards  upon  which  turn  the  rotating  plate 
and  the  driving-pulley.  The  former  should  be  cut  or 
turned  in  any  hard  wood  (preferably  mahogany),  of  a 
circular  shape,  rather  wider  at  the  base,  and  terminating 
in  a  square  piece  to  fit  the  mortise-hole  A,  in  the  base 
board.  The  height  of  this  standard  (exclusive  of  the 
square  portion  that  enters  into  the  base  board)  should 
be  7f  in.,  and  it  should  taper  from  2  in.  in  diameter  at 
the  bottom  to  I  in.  at  the  top,  as  shown  at  Fig.  15,  E.  A 
piece  of  stout  brass  tubing,  3£in.  long,  about  -Jin.  in 
diameter  (external),  and  \  in.  bore,  is  next  procured  and 
soldered  at  one  extremity  to  a  stirrup-shaped  piece  ot 
brass  made  out  of  A  in.  stuff.  This  is  first  cut  into  a 
long  oval,  i£  by  I  in.,  and  then  bent  up  in  the  middle  to 
admit  the  tube.,  'The  straight  piede  is  shown  at  F,  after 
bending  at  G,  and  with  the  piece  of  tube  soldered  in  its 
place  at  H.  Two  holes  are  drilled  and  countersunk  in 
the  projecting  brass  ears,  and  these  serve  to  screw  the 
tube  to  the  top  of  the  standard.  This  tube  forms  the 
"  sleeve"  in  which  the  spindle  that  supports  the  rotating 
disc  turns.  This  main  standard,  with  the  "sleeve"  screwed 
in  position,  is  figured  at  I.  Our  next  step  is  to  make 
the  two  lesser  uprights  which  support  the  driving-wheel. 
These  should  be  made  in  \  in.  stuff,  planed  up  nice  and 
smooth,  about  i-J  in.  wide  by  4^  in.  in  height  The  top 
of  each  standard  should  be  rounded,  and  a  slot,  about 
§  in.  wide  and  f  in.  deep,  cut  in  each.  One  of  these 
standards  is  shown  at  J,  along  with  the  small  piece 
which  is  placed  in  the  slot,  after  the  splindle  has 


HQL  TZ  MA  CHIN2.  49 

been  put  there.     A ,  little  pin,   run  through  laterally 


retains  the  whole  in  its  place.     Th.esc  two  uprights 
are  placed  facing  each  other,  at  B  and  B',  and  sufficient 


SO  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 

space  must  be  left  between  them  for  the  driving-wheel 
to  turn  freely.  The  driving-wheel  should  be  made  of 
wood,  preferably  turned  up  on  a  lathe,  and  grooved 
round  its  circumference  to  the  depth  of  a  £  in.  At  a 
pinch,  it  may  be  made  entirely  by  hand,  without  the  use 
of  a  lathe,  by  sav/ing  a  circular  disc,  6  in.  in  diameter, 
out  of  a  piece  of  \  in.  deal,  and  producing  a  groove  in 
the  circumference  by  means  of  a  red-hot  iron  rod  carried 
evenly  all  round  periphery.  The  central  hole  should  be 
square,  to  admit  of  a  square-shouldered  wooden  spindle 
being  glued  therein,  the  projecting  extremities  of  which 
must  be  shaved  down  and  rounded  so  as  to  run  in  the 
slots  of  the  lesser  uprights  J.  The  driving-wheel,  with 
its  spindle  and  handle  attached,  is  shown  at  K.  We 
may  now  proceed  to  m^unt  a  glass  disc  on  a  half-icel 
and  spindle,  in  the  manner  described  at  §  20.  This  disc 
should  be  12  in.  in  diameter,  and  be  fastened  at  its  exact 
centre  to  one  spindle  only,  as  illustrated  at  Fig.  15,  L. 
This  spindle  should  be  of  steel,  and  should  project 
beyond  the  half-reel  about  5  in.  ;  or,  in  other  words, 
the  distance  between  the  surface  of  the  disc  and  the 
extremity  of  the  spindle  should  be  about  6  in.  This 
cpindle  must  run  freely  (but  without  play)  in  the  sleeve 
at  the  top  cf  the  main  standard.  About  I  in.  of  the 
'  spindle  should  project  beyond  the  sleeve  when  the  reel 
end  is  quite  against  the  sleeve  at  the  other  extremity. 
,  Over  this  projecting  piece  is  placed  a  wooden  pulley, 
[about  I  in.  in  diameter,  and  this  is  keyed  on  to  the 
spindle  so  as  to  cause  it  to  rotate  rapidly  when  connected 
.  with  a  gut  band  to  the  driving-pulley  K. 


ffOLTZ  MAC  HIKE. 


§  3  r.  In    the    older   form  of   mach!n<£   as    already 
mentioned,  it  was  customary  to  employ  a  disc  for  the 


16 


FIG.  1 6. 


stationary  plate.     In  this  disc  were  cut  three  apertures — 
one  central^  to  admit  the  spindle  carrying  the  rotating 


52  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMA  TEVR*. 

disc,  and  two  lateral ones,  technically  called  "'windows," 
in  vvnich  were  placed  the  pieces  of  paper  that  serve  as 
inductors.  The  cutting  of  these  apertures  was  a  serious 
matter  for  amateurs.  The  plan  herein  recommended 
will  be  found  equally  efficacious  and  very  much  simpler 
in  execution.  A  square  sheet  of  glass,  16  in.  by  16  in.; 
is  cut  straight  across  the  middle,  so  as  to  divide  it  into 
two  equal  halves  16  in.  by  8  in.,  as  illustrated  at 
Fig.  16,  A.  When  these  two  halves  have  been  separated^ 
it  will  be  found  very  easy  to  cut  out  two  semi-ovals,  as 
shown,  large  enough  to  admit  the  passage  of  the  spindle 
and  reel  of  the  rotating  disc.  Anyone  can  cut  these 
semi-ovals  with  a  corrjmon  "wheel-cutter,"  or  they  may 
be  even  nicked  out  with  a  key  under  water.  Similar, 
but  longer  and  shallower  semi-ovals,  are  cut  out  at  about 
2  in.  each  side  of  the  central  aperture.  The  length  of 
these  cuts  should  be  about  4  in.,  the  depth  about  half  an 
inch,  so  that  when  the  two  cut  sheets  are  placed  in 
juxtaposition  there  will  be  seen  a  central  aperture  about 
:t  in.  by  i£  in.,  flanked  on  either  side  by  oval  apertures, 
4  in.  by  I  in.  The  two  half  sheets  are  then  cemented 
together  in  position  by  means  of  two  glass  strips, 
1 6  in.  by  2  in.,  which  are  glued  to  the  two  sides  of  the 
divided  sheet  with  boiled  and  hot  Canada  balsam,  the 
said  sheet  being  laid  on  a  flat  table,  the  strips  loaded 
with  weights,  and  left  until  the  balsam  is  set,  which  will 
take  three  or  four  days.  The  general  appearance  of  the 
finished  fixed  sheet,  with  its  "  windows,"  central  aperture, 
and  side  strips  (the  position  of  which  is  indicated  by 
dotted  lines),  is  sufficiently  well  shown  at  Fig.  16,  A. 


HOLTZ  MACHINE. 


S3 


*  32.  This  'plafe,  when  quite  set  and  firm,  may  have 
any  excess  6r  Canada  balsam  that  may  have  exuded 
under  pressure  scraped  off  and  cleaned  away  with  a  rag 
moistened  with  benzoline.  It  should  then  be  placed  on 
the  base  bpjard,  as  shown  at  Fig.  16,  B,  and  the  cleats 


FIG.  17." 

/and  d ^fastened  thereto  by  glue  and  screws,  the  heads 
of  which  must  be  covered  with,  guttapercha.  The  plate 
must  not  be  permanently  fixed  to  the  board  until  after 
the  glass  pillars  have  been  inserted  ;  but  it  is  convenient 
to  place  it  between  the  cleats  (as  shown)  to  try  whethci 


54   ELECTRICAL  INSTRUMENT  MAKING  FOR  AMA  TEURS. 

the  standard  carrying  the  sleeve  in  which  the  spindle  of 
the  movable  plate  turns  is  at  the  right  height  and 
position.  This  also  enables  us  to  try  whether  the  half- 
reel,  which  is  cemented  to  the  movable  glass  disc  and 
spindle,  allows  the  said  movable  disc  to  come  near  enough 
to  the  fixed  plate  without  actually  touching  it.  The 
distance  between  the  two  plates  should  be  about  %  in. 
If  much  more,  the  brass  sleeve  may  be  filed  down  until 
the  distance  is  sufficiently  diminished ;  if,  •  on  the 
contrary,  the  disc  runs  too  near  the  fixed  plate,  a  brass 
washer  of  sufficient  thickness  may  be  adjusted  between 
the  end  of  the  sleeve  and  the  half-reel.  It  is,  perhapsi 
as  well  to  point  out  that  the  side  of  the  glass  sheet 
facing  the  rotating  disc  must  be  the  one  on  which  the 
strips  are  not. 

To  insure  good  effects,  the  glass  plates,  as  well  as  the 
"  armatures "  or  projecting  paper  combs,  which  act  as 
•"inductors"  through  the  windows,  must  be  carefully 
coated  with  good  shellac  varnish.  Shellac  dissolved  in 
methylated  spirit  is  the  best  for  this  purpose. 

§  33.  The  armatures  must  now  be  placed  in  position. 
These  consist  in  two  portions  of  the  periphery  of  a 
circle,  cut  out  of  stout  paper,  as  illustrated  at  Fig  16,  C. 
The  scalloped  portion  must  project  through  the  windows, 
so  as  to  just  graze  the  surface  of  the  movable  plate. 
The  armatures  are  fastened  on  the  "  back  "  of  the  fixed 
plate  (the  side  farthest  from  the  rotating  disc)  one  above 
one  window,  and  the  other  below  the  opposite  window, 
by  means  of  good  starch  paste,  and  the  whole  carefully 
varnished,  as  above  recommended,  when  dry.  This 


BOLTZ  MACHIN&,  *$ 

having  been  satisfactorily  performed,  two  glass  standards, 

about  8  in.  in  height,  and  £  in.  thick,  are  fitted  with 

brass  tube  tops,  to  which  have  been  previously  soldered, 

two  cross  arms,  shaped  like  the  letter  T.    The  tops  can 

be  securely  fastened  to  the  glass  rods  by  means  of  plaster 

of  Paris.     Along  the  top  of  each  T  are  soldered  a  dozen 

brass  brads  to  serve  as  combs  to  collect  the  induced 

electricity.     To  the  leg  of  each  T,  beyond  its  junction 

with  the  brass  tube  that  fits  on  the  glass  rod,  is  soldered. 

a  hollow  brass  ball  about  ij  in.  in  diameter,  paving  a 

i  in.  hole  right  through  it     This  hole  serves  for  th* 

passage  of  a  sliding  brass  rod,  furnished  with  a  glass 

handle  on  the  outer  extremity;  and  an  inch  brass  ball  ?-.t 

the   other.     The  outer  extremity  of  each  head  of  tha 

T-piece    must    be    furnished    with   a  smooth  wooden 

prolongation  at  right  angles  with  the  T.     This  has  a 

deep  "  nick  "  in  its  inner  surface,  and  serves  to  support 

the  fixed  plate.     The  combs  at  the  end  T-pieces  must  be 

at  such   a  height    that   they  come  just   opposite    the 

scalloped    edges    of   the    paper    armature.      A     glass 

standard,  with  its  comb,  ball,  sliding  rod,  and  projecting 

wooden  arm,  is  illustrated  at  Fig.  17,  A. 

After  the  rotating  glass  plate  has  been  placed  in 
position,  with"  its  spindle  in  the  sleeve,  and  the  small 
driving  pulley  keyed  or  screwed  at  the  other  end,  the 
glass  pillars  may  be  cemented  in  position  with  Prout's 
elastic  glue,  care  being  taken  that  the  nicks  in  the 
wooden  prolongation  of  the  T-pieces  come  into  firm 
contac.t  with  the  fixed  glass  plate.  A  stout  gut  band 
will  be  found  best  to  drive  with.  The  complete  instru- 


56  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 

ment  is  shown  at  Fig.  17,  B.  It  is  usual  to  add  Leyden 
jars  to  these  instruments,  as  in  the  Bertsch  and  Carr<$ 
machines,  in  order  to  increase  the  capacity  of  the  two 
conductors. 

N.B. — The  Canada  balsam  for  attaching   the  strips 
to  the  plates  is  best  thickened  by  placing  in  a  shallow 


FIG.  18. 

saucer  in  a  slow  oven,  and  gently  heating  it  until  a 
small  portion  withdrawn  on  a  cold  iron  rod,  sets  ivJten 
cold.  It  must  be  applied  while  warm,  and  the  glasS 
strips  should  also  be  warmed. 

It  is  with  considerable  diffidence  that  the  author 
ventures  to  place  before  the  reader  the  following  in- 
structions concerning  the  construction  of  the  Wims« 


\TIIE  WJMSHURST  INFLUENCE  MACHINE  57 

hurst  machine,  since  the  machine  has  been  so  fully  and 
ably  described  by  the  inventor  himself.  However,  to 
render  this  series  complete,  a  brief  sketch  of  the  mode 
of  making  two  simple  forms  will  be  given. 

§  34.  JHE  WIMSHURST  INFLUENCE  MACHINE. — The 
portions  that  first  demand  cur  attention  are  the  plates. 
These  should  be  of  glass — good  window-glass — as  flat 
as  can  be  got,  and  not  too  green  in  colour  (as  it  is  apt 
to  be  poor  in  insulating  power)  is  to  be  preferred.  This 
is  to  be  cut  into  two  discs,  each  16  in.  in  diameter. 
The  thickness  of  these  discs  should  not  exceed  ^th 
of  an  inch.  As  in  the  Carr^  and  Bertschmachines.it 
will  be  actually  better  not  to  have  holes  drilled  in  the 
plates,  but  to  fasten  the  bosses  to  the  plates  as  described 
at  §  20.  These  bosses  consist  in  two  circular  pieces  of 
mahogany  or  other  well-seasoned  wood,  not  less  than 
4  in.  in  length.  (It  is  a  great  mistake 'to  have  these 
too  short,  as  then  the  glass  discs  come  too  near  the 
standards,  and  much  electricity  leaks  away.)  It  is  best 
to  turn  these  up  in  the  lathe,  of  the  form  ajid  dimen- 
sions shown  in  Fig  iB,a.  While  in  the  lathe,  a  perfectly 
central  hole  nearly  \  in.  in  diameter  must  be  bored  in 
the  small  end  of  the  boss  (as  shown  in  the  cut),  reaching 
nearly,  but  not  quite,  to  the  thick  end  of  the  boss.  This 
hole  must  be  bushed  for  its  whole  length  with  \  in.  brass 
tubing,  and  some  stout  steel  wire  which  just-enters  the 
brass  tube,  selected  and  straightened  out  to  serve  as' 
spindles  on  which  the  bosses  are  to  turn.  Mr.  Wims-' 
hurst,  in  his  directions,  says  :  "  I  must  impress  upon 
those  who  make  a  machine  that  they  cannot  give  too 


58  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 

much  care  in  selecting  the  tubes,  to  fit  properly,  on 
the  steel  wire ;  it  will  save  trouble  throughout  the 
making,  and  the  machine,  when  made,  will  work  more 
smoothly."  The  centre  of  the  gla'ss  plates  having  been 
obtained  as  described  in  §  20,  the  bosses  are  attached 
to  the  plates  precisely  as  described  therein.  When  quite 


set  and  dry,  the  plates  should  be  varnished,  in  a  warm, 
dry  room,  on  both  sides,  with  good  shellac  varnish,  and 
the  varnish  dried  by  the  fire. 

§  35.  As  shellac  varnish  is  in  constant  requisition  for 
insulating  electrical  apparatus,  I  give  Mr.  Wimshurst's 
recipe  for  the  preparation  and  preservation  of  this  useful 
compound.  Take  a  large,  wide-mouthed  bottle— say, 


THE  WIMSHURST  INFLUENCE  MACHINE.  59 

a  pickle  bottle— fit  to  it  a  soft  wooden  bung,  bore  a  hole 
through  this  bung,  and  in  this  hole  tightly  fix  the 
handle  of  a  rather  large  brush  (the  brush  end  being  in 
the  bottle) ;  then  about  half-fill  the  bottle  with  good 
shellac,  cover  the  shellac  with  r  methylated  spirits,  and 
shake  the  bottle  occasionally.  In  about  24  hours  it 
will  be  ready  for  use.  By  this  means  the  brush  is 
always  clean  and  serviceable. 

§  36.  While  the  varnish  is  drying  on  the  glass  discs 
the  operator  may  strike  out  on  a  piece  of  paper  a  circle 
of  the  same  diameter  as  the  discs,  and,  by  means  of  the 
compasses,  divide  the  circle  into  sixteen  equal  parts, 
drawing  radial  lines  at  each  division,  from  centre,  to 
circumference.  This  divided  circle  will,  if  placed  on 
a  .flat  table  under  the  glass  discs,  enable  the  operator 
to  fasten  down  the  tinfoil  "  sectors "  or  segments,  at 
equal  distances  from  each  other. 

§  37.  These  sectors  consist  in  wedge-shaped  strips  of 
tinfoil,  slightly  rounded  at  the  top  and  bottom,  as 
shdwn  at  b  (Fig.  18),  3^  in.  in  length,  by  £  in.  wide  at 
top,  diminishing  to  J  in.  wide 'at  bottom.  These  sectors 
are  easily  attached  to  the  glass  plates  by  placing  these 
latter  in  turns  upon  the  paper  circle  (§  36),  and,  having 
rubbed  a  little  thick  shellac  varnish  over  one  surface 
of  the  tinfoil  sector,  placing  it  (shellac  side  downwards) 
on  to  the  glass  plate,  just  over  one  of  the  lines,  care 
being  taken  to  leave  about  \  in.  of  clear  glass  between 
the  periphery  of  the  disc  and  the  circle  of  sectors. 
The  line  showing  this  distance  had  better  be  struck 
out  with  the  compasses  on  the  paper,  as  the  perfect 


fu  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 

regularity  of  the  circle  of  sectors  adds  much  to  the 
neat  appearance  of  the  machine.  One  disc,  mounted 
on  its  boss,  and  fitted  with  sectors,  is  shown  at  Fig.  iS^c. 
When  the  sectors  are  firmly  stuck  down  to  the  glass, 
and  the  varnish  quite  dry,  it  will  be  well  to  run  a 
brush  charged  with  varnish  round  the  inner  and  outer 
extremities  of  the  tinfoil  sectors.  These  rings  of 
varnish  may  extend  £  in-  inwards,  but  not  more.  They 
serve  to  increase  the  adherence  of  the  sectors  to  the 
glass,  ano!  a^so  to  insulate  slightly  the  extremities.  On 
the  centre  of  each  disc,  exactly  opposite  the  bosses, 
must  now  be  fastened  with  hot  marine  glue  or  Prout's 
elastic  glue,  a.  small  ebonite  washer,  punched  out  of 
sheet  ebonite  ^i  in.  thick.  These  washers  are  to 
prevent  the  rotating  glass  discs  from  actually  touching 
during  rotation. 

§  38.  The  -  stand  next  demands  our  attention.  It 
should  be  made  of  mahogany,  walnut,  or  some  other 
well-seasoned  wood.  Six  pieces  will  be  required — viz., 
two  pieces  20  in.  long  by  3  in.  wide,  and  I  in.  thick  ;  two 
pieces  14  in.  long  by  3  in.  wide,  and  two  uprights  17  in. 
in  height,  3.  in.  square.  These  two  latter  must  be  cut 
round  the  lower  end  to  form  a  square  tenon  2  in.  long 
by  2  in.  square  section,  and  two  of  their  sides  must  be 
made  to  slope  away  2  in.,  this  forming  the  top  end, 
which  must  be  rounded,  as  shown  at  Fig.  18,  d  d '.  A 
hole  of  exactly  the  diameter  of  the  steel  spindle  on 
which  the  plates  are  to  run  must  be  drilled  through  each 
standard,  at  about  an  inch  from  the  top,  care  being 
taken  that  these  two  holes  come  exactly  opposite  each 


THE  WIMSHVRST  INFLUENCE  MACHINE.  6l 

other  and  at  the  same  Jteight  in  the  standards,  otherwise 
the  plates  will  not  run  opposite  each  other.  At  about 
3f  in.  from  the  tenon,  and  on  the  same  side  of  each 
standard,  a  long  semi-cylindrical  slot  is  cut,  about  \  in. 
deep ;  this  serves  for  the  spindle  of  the  driving-wheels 
to  run  in.  All  these  pieces  are  shown  in  Fig.  1 8,  where 
d d'  are  the  uprights,*  e'  the  long  cross  pieces,  and//" 
the  lower  pieces  of  the  stand  ;  these  pieces  must  be 
planed  up  so  as  to  fit  accurately ;  mortice  holes,  cut  in 
the  centre  of  the  20  in.  strips  to  take  the  tenons  of  the 
uprights.  The  whole  is  then  joined  together  so  as  to 
make  a  strong  frame,  with  glue  and  screws ;  the  long 
strips  being  screwed  over  the  14  in.  pieces  at  each 
extremity,  so  as  to  form  a  square  14  in.  by  20  in.  As 
the  two  short  pieces  are  placed  below  the-  larger  ones, 
and  as  the  uprights  are  morticed  into  these,  it  is  neces- 
sary to  make  the  ends  of  the  uprights  project  I  in. 
through  the  holes,  so  as  to  afford  a  support  to  the  centfe 
of  the  frame.  With  a  spokeshave,  or  similar  tool,  the 
sharp  edges  should  be  taken  off  the  frame,  so  as  to 
prevent  dispersion  of  electricity.  The  frame  and 
standard  being  put  together,  as  shown  at  Fig.  19.  A,  it 
will  be  well  to  cut  out  the  holes  in  the  centre  of  the 
shorter  pieces  of  the  frames,  which  holes  are  to  receive 
the  glass  rods  or  jars  (as  the  case  may  be)  which  support 
the  conductors  and  combs.  These  holes  may  be  begun 
with  a  centre-bit,  and  should  be  2  in.  in  diameter  if  jars 
are  to  be  used,  or  I  in.  if  glass  rods  only  are  to  be 
employed. 

§  39.  As  the  glass  discs,  when  mounted,  must  rotale 


62  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 

in  contrary  directions,  it  is  necessary  to  have  two 
driving-wheels  on  a  spindle  wherewith  to  drive  them, 
and  to  connect  one  driver  with  one  boss  (§  34)  by  means 
of  a  straight  band,  while  the  other  transmits  its  motion 
by  means  of  a  crossed  band.  These  driving-wheels 
may  be  turned  out  of  any  suitable  wood,  7^  in.  in 
diameter;  they  should  have  a  centre-bit  hole,  \\  in. 
in  diameter,  put  through  the  centre ;  a  length  of  some 


FIG.  20. 

good  hard  wood  should  then  be  turned  up  to  make 
the  spindle,  on  which  the  two  wheels  must  be  tightly 
fitted  and  glued.  Care  must  be  taken  that  the  edges 
of  the  wheels,  when  glued  to  the  wooden  spindle,  should 
come  exactly  opposite  the  V  groove  in  the  bosses  (§  34) 
when  the  glass  discs  are  in  their  places.  These  wheels 
must  have  a  groove  turned  in  their  edges,  to  take  the 
driving-band.  The  length  of  the  wooden  spindle  must 


THE  WIMSHURST  INFLUENCE  MACHINE.  63 

be  such  as  to  just  not  reach  from  standard  to  standard 
say  "j\  in.  A  central  hole,  about  \  in.  in  diameter,  js 
now  made  through  the  entire  length  of  this  wooden 
spindle,  and  through  this  wooden  spindle  is  driven  an 
iron  rod  15^  in.  in  length,-  \  in.  diameter,  projecting 
3  in.  at  one  end,  and  4  in.  at  the  other.  At  the  longest 
end  this  rod  is  squared  up  to  take  a  driving-handle. 
If  the  wooden  spindle  docs  not  fit  quite  tight  upon  the 
metal  rod,  it  will  be  better  to  drill  a  hole  through  wood 
and  iron  and  drive  in  a  metal  pin.  The, driving  spindle 
is  placed  in  the  two  semi-cylindrical  slots  cut  in  the 
sides  of  the  standards  (§  38),  and  is  kept  in  its  placevby 
means  of  two  small  rounded  cleats  screwed  thereto. 
These  driving-wheels,  with  their  spindles,  etc.,  are 
represented  at  Fig.  19,  -B. 

§  40.  The  next  step  is  to  mount  the  comBs  and  con- 
ductors on  to  the  glass  pillars,  which  are  to  insulate 
them.  These  pillars,  of  which  two  are  required,  should 
be  I  in.  in  diameter  and  15  in.  in  length.  Good  white 
glass  that  becomes  readily  electrical  on  friction  should 
be  chosen.  They  must  be  fitted  with  brass  collars  I  in. 
long,  at  the  upper  extremities,  and  to  these  collars  are 
soldered  brass  balls  2  in.  -in  diameter.  At  the  upper 
portion  of  these  balls  is  drilled  a  hole  \  in.  in  diameter, 
into  which  are  fitted,  but  not  fixed,  brass  rods  \  itl.  thick, 
about  1 6  in.  long.  These  rods  are  bent  into  quadrants, 
and  terminate  in  brass  balls.  It  must  be  noted  that 
the  two  brass  balls  terminating  these  rods  must  be  of 
different  sizes  to  obtain  the  best  effects.  In  the  sized 
instrument  under  consideration,  one  should  be  f  in.  an4 


64  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 

the  other  I  £  in.  in  diameter,  and  nicely  polished.  By 
not  having  the  rods  as  fixtures  in  the  top  of  the  glass 
pillars,  it  is  easy  to  exchange  the  rods  if  the  direction  of 
the  flow  of  the  current  in  the  machine  is  reversed,  either 
accidentally  or'  intentionally.  The  combs,  which  must 
be  screwed  to  the  centre  of  each  2  in.  ball  that  terminates 
the  glass  pillars,  consist  in  J  in.  brass  rods  12  in.  long, 
terminating  in  brass  balls  about  £  in.  diameter  and  bent 
into  the  shape  of  a  horseshoe,  so  as  to  embrace  both 
the  plates.  In  the  interior  of  the  horseshoe  are  soldered) 
or  otherwise  affixed,  four  or  five  £  in.  brass  points.  The 
whole  arrangement  of  the  glass  pillars,  with  its  collecting 
comb,  large  ball,  movable  end,  and  terminating  ball,  is 
shown  at  Fig.  19,  C. 

§  41.  The  plates  can  now  be  mounted  on  the  stan- 
dards. To  this  end,  each  plate  in  turn  is  held  with  its 
boss  against  the  small  hole  at  the  top  of  the  standard 
on  the  steel  rod,  which  was  chosen  as  running  freely  in 
the  bushing  of  the  boss  (§  34),  pushed  through  this  hole 
and  into  the  hole  in  the  boss  as  far  as  it  will  go.  The 
rod  is  then  cut  off,  leaving  an  inch  projecting  on  the 
outside  of  the  standard.  With  a  file,  a  nick  is  cut  in  this 
steel  rod  at  about  the  centre  of  the  portion  that  is  to 
remain  in  the  standard,  then  a  round-headed  screw  is 
driven  into  the  top  of  the  standard  to  enter  this  nick  and 
keep  the  steel  rod  quite  firm  and  immovable.  To  the 
projecting  ends  of  the  steel  rods  are  affixed  the  bent  rods 
carrying  the  brushes.  These  rods  are  technically  known 
as  the  "neutralising  rods."  They  are  made  from  J  inJ 
brass  tod,  about  17  in.  in  length.  A  hole  J  in.  deep  ant^ 


THE  PLATE  ELECTRIC  MACHINE.  t5 

about  |  in.  wide,-is  drilled  at.  the  two  extremities  of  each 
of  these1  rods.  A  little  tuft  of  about  a  dozen  short 
lengths  of  the  fine  wire  used  by  the  gilt  lace  manufac- 
turers, is  made  into  a  brush  by  binding  at  the  bottom 
end  with  a  ijttle  of  the  same  wire ;  this  same  end  is 
pushed  in  the  holes  in  brass  rod,  and  wedged  firmly  into 
place  by  means  of  a  little  wooden  wedge.  There  are 
two  such  rods  ;  and  each  rod  must  have  a  brush  at  eacn 
end.  A  short  length  of  brass  tubing,  fitting  tightly  on 
to  the  projecting  pieces  of  the  steel  spindles  carrying  the 
plates,  is  now  procured,  and  cut  into  two  pieces  \\  in.' 
long.  With  a  round  file,  a  slot  (sufficiently  deep  to 
take  the  brass  neutralising  rods  just  finished)  is  produced 
in  these  pieces,  and  each  rod  having  been  placed  in  this 
slot,  at  its  centre,  is  soldered  thereto.  At  the  same  point, 
£o  give  a  finish,  and  prevent  dissipation  of  electricity,  is 
also  soldered  a  small  brass  ball.  Each  rod  is  then  bent 
slightly  in  the  shape  of  a  bow,  so  that  the  "  brushes  " 
'shall  come  into  contact  with  the  sectors,  when  the 
neutralising  rods  are  put  in  their  places  on  the  steel 
spindle. 

The  appearance  of 'the  comclete  machine,'  with  the 
"brushes"  in  their  right  position  for  a  right-handed 
person  to  drive,  is  shown  at  Fig.  2O/ 

§  42.  THE  PLATE  ELECTRIC  MACHINE.— The  ordi- 
nary frictional  machine,  although  far  behind  the  induc- 
tion machines  in  point  of  efficiency,  will  always  be  a 
favourite  with  amateurs,  owing  to  the  ease  with  which  it 
may  be  constructed. 

A  circular  glass  plate   12  in.  in  diameter,  having  been 


1V>  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 

cut  but  and  mounted  by  either  of  the  methods  described 
at  §  iSet seq,  is  supported  by  its  spindle,  on  two  standards 
(similar  to  those  described  and  figured  at  §  22),  standing 
about  8  in.  high,  2  in.  wide,  and  £  in.  thick!  v  These  are 
let  in,  by  mortising,  to  a  base  board  14  in.  by  9  in.  by 
I  in,,  at  a  distance  of  about  2  in.  from  each  other,  with 
their  flatter  sides  facing  as  shown  at  §  39,  with  this 
difference — that  they  must  stand  within  an  inch.on  either 
side  of  the  centre  of  the  base  boarjd  with  the  glass  plate 
between  them.  The  wood  to  be  used  for  all  the  parts  of 
this  machine  should  be  thoroughly  well  seasoned,  and  if 
after  being  planed  up  it  is  well  rubbed  with  a  flannel 
pledget  dipped  in  melted  paraffin  wax,  it  will  be  much 
(improved%  The  spindle  on  one  side  should  not  project 
beyond  the  standard,  while  on  the  other  it  should  extend 
about  i  in.  beyond  the  opposite  standard,  and  on  this  end 
should  be  filed  up  square  to.  take  a  handle  by  which  it  is 
to  be  rotated.  As  in  §  19,  so  here,  a  shoulder  must  be 
'turned  at  each  end  of  the  spindle  to  prevent  the  plate 
riding  backwards  and  forwards  on  the  standards.^  Two 
cushions  are  now  made  by  cutting  two  thin  smooth 
pieces  of  wood  4  in.  long,  i£  in.  wide,  and  a  £  in.  thick, 
and  covering  these  on  one  side^with  tinfoil  attached 
with  good  paste'or  thin  glue.^  When  dry,  thej tin-foil  is 
.covered  over  to  the  depth  of  about  f  in.  with^  horsehair, 
'mixed  with  some  short  lengths  of  ^very  fine  iron  wire, 
such  as  is 'used  by  florists  under  the  name  of  "  binding 
wire."  A  square  of  flannel  of  the  same  size  as  the  little 
board  is  now  laid  over  the  packing,  and,*  finally,  the 
whole  is  covered  with  good  wash-leather,  which  is  drawn 


THE  PLATE  ELECTRIC  MACHINE.  67 

tightly  round  the  edges  and  ghied  thereto,  tacks  being 
used  to  hold  the  leather  in  its  place  until  the  glue  is  dry. 
N.B. — The  corners  of  the  wood  to  which  the  leather  is 
attached  should  be  raunded,  to  prevent  dissipation  of 
electricity. 

In  order  that  the  cushions  may  press  firmly  against 
theglass  plate,  and  yet  at  the  same  time  give  to  any 
inequality  in  surface,  or  want  of  elasticity,  it  is  well  not 
to  fix  the  rubbers  or  cushions  in  any  permanent  manner 
to  the  standards.  A  mode  which  works  admirably,  is  to 
drill  two  holes  at  the  centre  of  the  edge  of  each  cushion 
to  the  depth  ofabout  f  in.  A  stout  piece  of  brass  wire 
(say  \  in.  in  diameter)  IO  in.  in  length,  is  hammered  at 
its  centre,  to  render  it  springy,  and  then  bent  into  the 
shape  of  a  letter  U,  the  legs  standing  at  such  a  distance 
apart  as  just  to  face  the  two  holes  just  bored  in  the 
cushions  when  these  latter  are  held  firmly  against  the 
glass  plate.  The  top  of  each  leg  of  the  U  is  then  bent 
sharply  at  right  angles  to  the  rest  of  the  leg  for  a  length 
of  f  in.,  the  bend  in  each  leg  facing  and  being  parallel 
to  its  neighbour.  The  U  is  then  squeezed  together  at 
its  upper  extremities  until  the  two  bent  pieces  just 
touch. 

These  pieces  *~are  pushed  into  the  holes  in  the 
cushions,  care  being  taken  that  the  leathers  of  the 
cushions  face  one  » another.  The  plate  having  been 
placed  in  its  position  on  the  standards,  the  two  cushions 
(which  are  held  face  to  face  by  the  U  spring)  are  slightly 
separated,  and  caused  to  grip  the  glass  plate  between 
them.  They  must  be  pushed  so  far  towards  the  centre 


68  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 

of  the  glass  plate  as  to  clear  it  by  about  2  in.  The  plate 
is  allowed  to  turn  until  the  cushions  come  in  a  line  with 
the  standards,  and  then,  having  decided  in  which  direc- 
tion the  plate  is  intended  eventually  to  be  rotated,  three 
cleats  of  wood  are  glued  to  the  inside  of  the  standards 
to  prevent  the  cushions  from  being  carried  round  during 
rotation.  These  three  cleats  or  stops  are  put  together 
like  a  letter  E  without  the  central  stroke,  so  that  the 
mere  act  of  rotating  in  the  right  direction  causes  the 
cushions  to  hold  in  their  right  position  on  the  standards, 
while  the  U -spring  pinches  them  against  the  plate. 
When  it  is  desired  to  remove  the  cushions  for  the  pur- 
pose of  amalgamating,  etc.,  it  is  only  needful  to  give 
the  handle  a  half-turn  backwards,  when,  of  course,  the 
plate  being  gripped  by  the  spring  and  cushions,  brings 
these  along  with  it  out  from  between  the^  cleats.  A 
metallic  chain  should  hang  from  the  bottom  of  the  U- 
spring  and  touch  the  base  board. 

A  solid  glass  rod  16  in.  by  f  in.  in  diameter  must  now 
be  procured.  This  must  be  cemented  into  a  wooden 
foot  about  £  in.  thick  by  2  in.  wide,  and  3  in.  or  4  in. 
long,  having  a  rather  long  slit  through  it,  so  that  it  can 
be  screwed  down  to  the  base  board  of  the  electric 
machine  by  means  of  a  thumbscrew,  the  slit  being  in- 
tended to  allow  of  a  little  adjustment  in  the  way  of 
approach  or  recession  from  the  plate.  It  is,  perhaps, 
needless  to  observe  that  this  glass  rod  must  be  placed 
on  that  side  of  the  glass  plate  where  the  handle  is  not, 
just  in  a  line  with  the  standards.  This  glass  rod  must 
be  surmounted  with  a  brass  or  wooden  ball,  at  least  4  in. 


THE  PLATE  ELECTRIC  MACHINE.  69 

In  diameter  (a  good  skittle  ball  does  admirably).  If  of 
wood,  it  must  be  carefully  and  smoothly  covered  with 
tinfoil.  Besides  the  hole  into  which  the  glass  rod  enters 
and  to  which  it  must  be  cemented,  this  ball  has  two, 
other  holes — viz.,  one  about  J  in.  in  diameter  fadrrg  the 
glass  plate ;  another,  also  about  \  in.  in  diameter,  at  the 
top,  opposite  and  perpendicular  to  the  one  in  which  the 
glass  rod  enters.  Into  this  latter  is  fixed  a  rod  of  metal 
about  Jin.  in  diameter,  which  extends  straight  upright  for 
a  distance  of  4  in.  from  the  ball,  and  is  then  bent  so  as  to 
form  a  ring  12  in.  in  diameter.  This  ring,  and  the  rod, 
except  at  the  extremity,  where  it  enters  into  the  ball, 
should  be  carefully  and  neatly  wrapped  with  good  string, 
£  in.  thick,  after  the  manner  in  which  fencing  foil 
handles  are  wound.  The  coating  of  string  should 
bfe  rubbed  over  with  melted  paraffin  wax,  and  lastly, 
carefully  varnished  with  the  red  varnish  previously 
described. 

The  effect  of  this  ring  (known  as  Winter's  ring)  is 
greatly  to  intensify  and  lengthen  the  spark. 

All  that  now  remains  to  be  done  is  to  make  the 
collectors.  To  th;s  end,  two  rings,  4  in.  in  diameter, 
must  be  made  out  of  \  in.  galvanised  iron  wire,' allowing 
a  length  of  about  5  in.  of  wire  to  project  beyond  the 
circles.  A  dozen  sharp-pointed  brads  are  now  to  be 
Soldered  at  equal  distances  all  round  these  rings  so  that 
When  the  rings  are  lying  flat  on  a  table  the  points  of  the 
brads,  stand  upright.  The  projecting  wires,  or  stalks,  of 
these  rings  are  now  bent  sharply  at  right  angles  to  the 
tings,  at  about  \  in.  from  the  rings,  in  such  a  manner 


fo  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 

that  if  placed  with  the  stalks  in  the  side  hole  in  the 
ball,  the  teeth  in  the  one  r;ng  shall  face  the  surface  of 
the  glass  plate  nearer  the  handle,  while  the  teeth  of  the 
other  ring  shall  face  that  surface  of  the  plate  which  is 
nearer  the  ball. 

Having  adjusted  the  rings  so  that  the  distance 
between  the  teeth  of  the  rings,  and  the  glass  plate  shall 
not  be  more  than  the  £  in.,  nor  less  than  \  in.,  the 
operator  will  solder  the  two  stalks  together  ;  cut  them 
off  to  such  a  length  as  to  enable  them  to  enter  the  hole 
in  the  ball,  and  yet  retain  the  correct  distance  either 
side  of  the  plate.  The  stalks  should  then  be  wrapped 
in  tv/ine  (except  at  the  end  that  enters  the  ball),  paraf- 
fined and  varnished  as  above.  This  end  may  now  be 
glued,  wrapped  in  tinfoil,  and  forced  tightly  in  the  hole 
in  the  ball.  Such  a  machine,  with  cushions  freshly 
dressed  with  amalgam,  will  give,  in  dry  weather,  sparks 
2  in.  long. 

§  43.  CONDENSERS. — These  are  instruments  which 
owe  their  peculiar  power  of  retaining  charges  of  electricity 
to  the  fact  that  one  conducting  surface  connected  to 
earth  or  other  large  conductor  confers,  by  induction, 
a  greater  capacity  for  .charge  in  a  second  insulated 
conducting  surface  than  this  latter  would  have  were  it 
not  for  the  presence  of  the  first  surface. 

Condensers  may  be  conveniently  divided  into  three 
groups,  according  to  their  forms,  and  the  nature  of  the 
insulator  used  between  the  two  conducting  surfaces — 
1st,  Bottle  formed,  or  "  Leyden  jars";  2nd,  Coated 
glass  sheets,  also  called  fulminating  panes  or  Franklin's 


THE  LEYDENJAR.  ft 

plates  ;  3rd,  Coated  sheets  of  paraffined  paper  or  other 
flexible  insulator,  known  as  Fizeau's  condensers. 

§  44.  THE  LEYDEN  JAR. — Every  dabbler  in  elec- 
tricity has,  at  one  time  or  other,  made  a  Leyden  jar ; 
not  every  one  has  made  a  good  one.  The  first  point 
to  be  noted  is  the  shape  of  the  bottle  or  jar  to  be  used. 
It  must  have  a  neck  sufficiently  wide  to  allow  the  inner 
coating  to  be  easily  placed  in  it  and  smoothly  laid. 
To  this  end  the  mouth  should  certainly  not  be  less 
than  2  in.  in  diameter  for  a  jar  6  in.  by  3  in.,  and  so  on 
in  proportion.  The  next  point  that  demands  attention 
is  that  the  bottle  or  jar  should  be  of  nearly  uniform 
thickness,  and  free  from  flaw  or  crack.  To  ascertain 
this,  it  should  be  made  to  "  ring."  The  last,  but  certainly 
most  essential  point,  electrically  speaking,  is  that  the 
glass  of  which  it  is  constructed  should  be  really  a  good 
insulator.  The  white,  or  rosy  white  glass  (which  con- 
tains rnanganese)  is  very  deficient  in  this  regard.  The 
greener-coloured  glasses  are  generally  good  insulators. 
To  test  this  point  clean  and  dry  the  bottle.  When 
quite  dry,  but  cold,  rub  it  briskly  on  the  outside  with 
a  warm  silk  handkerchief.  Reject  all  such  jars  as  do 
not  easily  and  quickly  become  charged  so  as  to  give 
a  distinct  spark.  Choose  all  such  as  retain  the  charge 
(once  acquired)  longer  than  the  others.  The  jar  or  jars 
chosen  should  now  be  fitted  with  a  sound  bung.  This 
requires  cutting  neatly  above  and  below,  so  as  to  be 
quite  smooth,  the  upper  surface  being  afterwards  rather 
thickly  coated  with  good  red  sealing-wax.  (N.B. — 
The  coating  of  sealing-wax  should  not  be  applied 


72  ELECTRICAL  INSTRUMENT  MAKING  FOR  A  MA  TEURS. 

until  the  bung  has  been  pierced  centrally  and  a  rod 
fitted.)  ; 

It  is  essential1  that  the  bung  should  be  new,  or  at 
least  unused,  for  if  it  has  served  for  acid  or  salts,  it  will 
be  found  to  attract  moisture,  and  greatly  detract  from 
the  efficiency  of  the  jar.  The  knob  of  the  jar  should  be 
one  of  those  brass  balls  which  can  bs  procured  from 
most  of  the  metal-shops  in  Clerkenwell,  where  they  are 
known  under  the  name  of  "  metal  beads."  A  female 
screw  can  be  made  in  the  orifice  of  the  brass  ball,  and 
a  thread  put  on  a  short  length  (say  6  in.)  of  stout  brass 
wire  to  fit.  This  "wire,  or  rod,  with  its  accompanying 
ball  is  now  fitted  to  the  bung.  This  is  effected  by 
making  a  small  central  hole  in  the  bung  with  a  bradawl, 
considerably  smaller  than  the  rod.  This  latter  is  then 
forced  in  until  about  two-thirds  of  its  length  has  been 
pushed  through  the  bung.  When  this  has  been  done, 
the  upper  surface  of  the  bung  (that  nearer  the  brass 
ball)  should  receive  its  coating  of  sealing-wax.  Should 
there  be  any  difficulty  in  procuring  a  brass  ball,  a 
substitute,  quite  as  efficient,  though  not  so  imposing 
in  appearance,  will  be  found  in  a  leaden  bullet,  cast 
upon  the  end  of  a  brass  or  copper  rod  of  the  desired 
diameter  and  length.  The  ne:ct  step  is  to  coat  the  jar 
with  tinfoil.  It  is  better  to  begin  with  the  inside,  as 
when  the  outside  is  coated  it  b  not  easy  to  see  what  is 
going  on  inside.  A  piece  of  good  tinfoil  is  cut  intD 
a  circle,  a  trifle  narrower  than  the  outside  diameter  of 
the  bottom  of  the  jar.  One  side  of  this  is  carefully 
rubbed  over  (\yith  the  finger)  with  thin  hot  glue.  It 


THE  LEYDEN  JAR.  73 

is  then  allowed  to  fall  in  the  jar,  glue  side  downwards, 
und  is  pressed  into  its  place  at  the  bottom  by  means 
of  a  small  mop  (similar  to  those  used  to  clean  lamp- 
glasses).  The  jar  is  now  measured  from  the  bottom  to 
the  bung.  Three-quarters  of  this  are  marked  off  as  the 
height  to  which  the  tinfoil  coating  may  reach.  A  strip 
of  tinfoil  of  this  width,  and  three  times  the  diameter  of 
the  jar,  is  now  cut.  For  convenience  of  getting  in  the 
jar,  especially  if  this  latter  be  narrow,  it  will  be  well  to 
divide  the  strip  of  tinfoil  into  two  halves,  and  having 
glued  one  half,  slip  it  in  the  jar,  having  previously  given 
it  a  little  bend,  push  it  into  its  place,  and  smooth  it; 
'down  by  means  of  a  tooth  brush,  then  insert 
the  other  half  in  a  precisely  similar  manner. 
Care  must  be  taken  that  these  side  pieces 
come  into  actual  contact  with  the  bottom 
circlet.  The  inside  being  thus  coated,  the 
outside  may  be  proceeded  with  in  precisely 
similar  manner  ;  only  the  sides  may  be 
covered  by  one  entire  piece  of  tinfoil,  and 
this  should  lap  somewhat  over  the  bottom. 
The  coatings  should  be  allowed  to  dry 
thoroughly  before  the  jar  is  used  or  touched, 
as  otherwise  the  outer  coating  will  be  wrinkled 
up  and  spoiled.  Very  charming  effects  of 
a  multiplicity  of  sparks  duripg  discharge,  may  .be 
obtained  by  coating  the  inside  of  the  jar  with  a  number 
of  small  diamonds  of  tinfoil,  almost,  but  not  quite 
touching,  instead  of  with  one  continuous  layer.  The 
outside  should,  in  this  case,  be  covered  with  similar 


I^LECTRICAL  INSTR  UMENT  MAKING  FOR  AM  A  TE  URS. 

diamond -shaped  pieces,  and  this  latter  should  have  a 
circular  hole  cut  in  each,  so  that  each  hole  should  stand 
over  the  corners  of  the  inside  diamonds.  By  this  means 
not  only  are  the  sparks  betv/een  the  outer  diamonds 
seen,  but  also  those  between  the.  inside  ones.  The  jar 
being  coated  and  dry,  the  bung,  with  its  rod  and  ball, 
is  fitted  to  it.  It  is  well  to  attach  a  small  piece  of  metal 
chain  to  the  end  of  the  rod  '(by  spidering),  to  insure 
contact  between  the  rod  and  the  inner  coating.  The 
absolute  size  of  the  jar  and  its  parts  may  be  varied  at 


Fns.  22. 

will  ;  but,  to  obtain  the  best  results,  the  following 
relative  dimensions  v/ill  bs  found  useful : — Let  the  height 
of  the  jar  be  three  times  its  diameter ;  the  length  of 
rod  and  ball  four  times  the  diameter.  With  these  pro- 
portions, and  especially  if  the  coatings  do  not  extend 
bsyond  thres-fourths  of  the  height  to  the  bung,  the  jar 
v/ill  be  found  to  hold  a  good  charge  for  some  time. 
A  lesser  charge,  with  longer  retention,  may  be  obtained 
by  using  less  coated  surface — say,  two-thirds,  or  even  a 
half,  of  the  total  height,  instead  of  three-fourths.  Fig.  21 


FULMINATING  PANES.  75 

shows  the  relative  sizes  and  shapes  of  the  different 
portions  of  the  jar  ;  Fig.  22  shows  the  inner  and"  outer 
diamond-shaped  pieces  of  tinfoil  to  be  used  if  a  "spangle 
jar"  be  desired. 

§45.  FULMINATING  PANES,  or""  Franklin's  plates"  as 
they  are  also  called,  are  easily  made  by  coating  both  sides 
of  a  sheet  of  glass  with  tinfoil,  to  the  extent  of  half  of  the 
entire  surface,  leaving  the  margins  all  round  clear  glass. 
The  glass  should  be  chosen  with  the  same  precautions 
as  to  insulating  power  as  in  the  jcase  of  Leyden  jars 


FIG   23. 

(§  44),  and  the  corners  of  the  tinfoil  should  be  rounded 
as  rounded  ones  dissipate  electricity  less  than  pointed 
ones.  Good  paste  or  glue  (thin)  will  do:  very  well  to 
stick 'the  foil  down  to  the  glass.^  When  quite  dry,  any 
superfluous  glue  or  pasfe  may  be  removed  from  the 
edges  of  the  foil  and  glass  by  means  of  a  slightly 
damped  rag.  The  tinfoil  should  be  lightly  burnished 
with  a  bone  or  ivory  knife  handle.  As  a  further 
protection  against  sparking  over,  it  is  well"  to  varnish 
round  the  edges  of  the  tinfoil  with  a  coating,  of  good 
shellac  varnish.  In  charging  this  condenser,  it  must  be 
borne  in  mind  that  one  surface  must  be  connected  to 
earth  or  other  large  conductor,  while  the  other  is. 


?6  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 

receiving  the  charge ;  otherwise  its  capacity  is  very 
limited.  A  very  convenient  size  and  form  of  the 
.Franklin  plate  is  shown  at  Fig.  23. 

§46.  Owing  to  the  facility  with  which  these  glass 
plates  are  broken,  they  are  not  much  used  at  present  as 
condensers.  Other  insulators,  having  at  once  the 
advantages  of  being  lighter,  less  fragile,  more  flexible, 
and  of  higher  specific  inductive  capacity  than  glass,  are 
now  used  in  all  instruments  intended  for-  practical  work. 
Of  these,  good  paper,  free  from  holes,  soaked  in  melted 
paraffin,  stands,  if  not  alone  as  the  first,  in  the  very  first 
rank.  Next  comes  paper  soaked  in  good  shellac 
varnish ;  then  thin  sheet  ebonite,  which  can  be  bent  by 
heat  to  any  shape  ;  and,  lastly,  good  indiarubber  cloth. 
The  mode  of  making  a  condenser,  with  either  of  these 
insulators,  will  be  the  same  in  each  case,  so  the  student 
will  do  well  to  try  his  hand  at  a  Fizeau's  condenser,  as 
being  at  once,  the  most  useful  and  least  expensive. 

§47.  FIZEAU'S  CONDENSERS. — These  are  employed  foi 
two  purposes :  (a)  to  increase  the  efficiency  of  induction 
coils,  by  taking  up  the  "  extra "  current  induced  in 
the  primary ;  (b)  to  measure  electrpstatic  capacity. 
Condensers  for  coils  may  be  of  any  dimensions  between 
9  in.  by  7  in.  and  2  in.  by  4  in.,  according  to  the  power 
of  the  instrument  with  which  they  are  intended  to  be 
used  ;  the  former  size  for  coils  giving  from  I  in.  to 
3  in.  spark,  the  latter  for  coils  giving  sparks  of  J  in.  or 
less.  In  the  former  case  as  many  as  150  alternations 
of  tinfoil  will  be  needed;  in  the  latter,  from  25  to  30 
v/ill  be  ample.  Having  decided  upon  the  size  to  be 


FIZEAU'S  CONDENSERS. 


77 


employed,  the  student  will  select,  at  any  photographic 
stores,  some  sheets  of  plain  paper  (not  salted  or 
albumenised)  known  as  Fapier  Rive.  These  sheets  run 
about  22* in. xi8  in.  so  that  each  sheet,  if  cut  in  half 
lengthwise,  and  in  three  across,  will  give  six  sheets 
7  in.  x  9  in.  In  selecting  this  paper  it  should  be  held 
between  the  eye  and  the  light,  so  that  any  holes  or 
imperfections  may  at  once  be  seen  ;  and  any  sheet  which 
is  faulty  in  this  regard  at  once  rejected.  The  paper 
having  thus  been  chosen,  and  cut  to  the  desired  size, 
should  be  placed  in  a  square  flat  dish,  a  trifle  larger  than 
the  'sheets  themselves.  The  best  dish  to  use  for  this 


FIG.  24. 

purpose  is  undoubtedly  one  of  the  square  white  earthen- 
ware dishes  in  which  photographers  sensitise  their 
papers  ;  but  if  one  of  these  is  not  procurable,  a  very  fair 
substitute  may  be  found  in  a  square,  flat,  tin  baking 
dish  made  scrupulously  clean.  The  sheets  having  been 
placed  in  the  dish,  several  lumps  of  good  paraffin 
wax  (one  requiring  a  very  high  temperature  for 
fusion  is  the  best  for  this  purpose)  are  scattered 
over  the  paper,  and  the  whole  stood  in  a  gentle  oven 
.until  the  wax  is  thoroughly  melted,  and  the  paper 


78  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 

has  become  semi-transparent  by  having  absorbed 
it.  If  there  are  any  patches  of  unsaturated  paper 
more  paraffin  -wax  must  "be  added,  until  the  paper 
is  equally  imbued  with  the  melted  paraffin.  When 
this  is  the  case,  the  sheets  of  paper  should  be 
removed  from  the  oven  one  by  one,  allowing  each  one 
to  drain  until  it  drips  no  longer.  The  paraffin  will  set 
almost  as  soon  as  each  sheet  is  removed  from  the  oven. 

The  next  operation  consists  in  placing  leaves  of  tinfoil 
in  alternation  with  the  paraffined  paper.  Supposing  we 
have  cut  our  paper  sheets  to  the  size  9  in.  by  7  in.,  we 
shall  require  about  1 50  sheets  of  tinfoil  7  in.  by  5  in. ; 
or,  in  other  words,  we  must  allow  a  clear  margin  of  I  ?n. 
of  paper  all  round  the  squares  of  tinfoil  We  shall  also 
need  150  strips  of  the  same  tinfoil  about  I  in.  wide  byj 
3  in.  long. 

Having  procured  two  thJn  pieces  of  board  (common 
deal  will  do)  of  the  same  sizes  aa  the  paper,  but  only  £  in. 
thick,  we  proceed  to  lay  upon  it  a.  sheet  of  paraffined 
paper.  In  the  centre  of  this  v/e  lay  a  sheet  of  tinfoil, 
and  on  this  sheet  of  tinfoil  we  place  one  of  the  3  in.  strips 
to  the  right  hand,  so  that  the  strip  projects  beyond  the 
paper  about  I  in.  We  now  place  another  sheet  Ot 
paraffined  paper  squarely  over  the  first  sheet,  and  then 
a  sheet  of  tinfoil  as  before.  On  this  second  sheet  of 
tinfoil  we  put  a  projecting  strip  of  tinfoil,  but  this  time 
to  the  left.  In  this  manner  the  condenser  is  built  up  of 
alternate  sheets  of  paper  and  tinfoil,  with  strips  of  tin- 
foil projecting  alternately  in  the  right  and  left.  In  our 
case,  all  the  odd  numbers,  I,  3,  5,  7,  etc,,  being  to  the 


MICROFARAD  CONDENSER  79 

right,  and  all  the  evens,  2,  4,  6,  8,  etc.,  being  to  the  left. 
Care  must  be  taken  to  end  the  condenser  with  a  sheet 
or  two  of  paraffined  paper.  The  other  board  should 
now  be  placed  over  all,  and  the  whole  bound  neatly 
and  tightly  together  by  Wrapping  round  crosswise  (not 
lengthwise  because  of  the  projecting  strips)  with  rather 
wide  tape  from  end  to  end,  and  then  stitching  down  the 
ends  of  the  tape.  This  being  done,  the  projecting  strips 
on  each  side  are  folded  tightly  together,  so  that  the 
whole  150  strips  come  into  absolute  contact.  It  is  to 
these  two  projecting  lugs  that  remain  after  this  opera- 
tion, that  the  contact  pillar  and  spring  of  the  coil  are 
attached.  Fig.  24  shows  the  disposition  of  the  sheets  of 


FIG.  25. 

paraffined  paper  and  tinfoil,  where  pp'  are  the  sheets  of 
paper,  and  TT7  the  tinfoil.  At  Fig.  25  is  shown  the 
complete  condenser,  with  the  projecting  tinfoil  strips 
turned  up  and  compressed  together. 

§  48.  MICROFARAD  CONDENSER.— Take  37  sheets  of 
good  tinfoil,  7  in.  by  6  in.,  with  an  equal  number  of  strips 
3  in.  by  \  in.  Procure  about  76  sheets  of  very  thin,  hot- 
pressed  paper  9  in.  by  8  in.,  such  as  is  used  for  bank- 
notes, or  similar.  Choose  it  carefully,  leaf  by  leaf,  free 
from  holes  or  blemishes.  Soak  it  as  above  described  in 
hot  melted  paraffin,  drain,  and  blot  off  each  individual 


8o  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 

sheet  between  good  blotting  paper.  A  hot  iron  must 
be  used  to  do  this  effectually.  No/w  lay  on  a  perfectly 
smooth  and  flat  iron  plate  two  sheets  of  this  paper  (as 
shown  at  P,  Fig  24),  then  a  sheet  of  tinfoil,  quite  in  the 
middle,  then  a  strip,  projecting  to  the  right.  Over  this 
are  placed  two  sheets  of  paraffined  paper  (as  shown  at 
P'),  then  a  sheet  of  tinfoil  with  the  projecting  strip  to  the 
left  (as  shown  at  T").  Proceed  in  this  manner,  laying 
the  sheets  of  tinfoil  over  every  two  sheets  of  paper  until 
the  37  sheets  have  been  used  up.  Now,  subject  the 
whole  in  a  press  between  two  iron  plates  to  a  pressure 
of  half  a  ton.  Solder  lightly  all  the  19  ends  on  the 
right-hand  side  together  for  attachment  to  one  terminal, 
and  the  18  ends  on  the  left-hand  side  to  attach  to  the 
other  terminal ;  place  the  condenser  between  two  stout 
boards  clamped  together  by  screws,  so  as  to  maintain 
one  constant  and  invariable  pressure  as  the  capacity 
increases  by  pressure.  Two  binding  screws  must  bs 
attached  to  the  two  soldered  ends  of  the  tinfoil  strips 
to  serve  as  terminals.  By  using  two  sheets  of  paper 
between  each  pair  of  tinfoils,  any  liability  to  leakage 
through  the  holes  in  the  paper  is  minimised ;  of  course, 
the  instruments  should  be  tested  against  one  of  known 
capacity  if  great  exactitude  be  required.  If  too  small, 
increase  the  pressure,  or  the  number  of  tinfoil  and  paper 
leaves  ;  if  too  great,  remove  a  leaf  or  two. 


PART  II. 
DYNAMIC  OR  CURRENT  INSTRUMENTS. 

§  49.  THE  MEDICAL  COIL. — Procure  a  well-seasoned 
board  of  walnut  about  21$  in.  in  length,  3  in.  wide,  and 
f  in.  thick.  From  this  cut  one  length  12  in.  long  for 
the  base  board  (Fig.  30),  and  three  pieces  3  in.  square 
(like  Fig.  29),  for  the  coil  heads;  when  cut,  a  fillet  8  in. 
long  must  be  nailed  or  screwed  on  the  two  sides  of  the 
base  board  (as  shown  at  Fig.  30)  ;  these  fillets  should  be 
in.  square  section.  Corresponding  square  nicks  must 
be  cut  in  two  of  the  square  heads  (as  shown  at  a}  a,  a,  a, 
Fig-  31)-  All  the  woodwork  when  thus  squared  and 
finished,  should  be  soaked  for  a  quarter  of  an  hour  in 
melted  paraffin  wax,  and  then  rubbed  dry  while  still 
warm. 

Procure  a  thin  brass  tube  (knovrn  in  the  trade  as 
"  triblet  tubing  ")  about  $  in.  diameter,  4^  in.  long  ;  turn 
up  a  short  plug  and  button  to  fit  one  end  of  this  tube 
and  serve  as  a  handle  (see  Fig.  26).  This  may  be 
fastened  to  the  tube  by  driving  in  three  fine  brass  brads, 
and  filing  off  the  heads  flush  with  the  tube. 

Now  cut  up  about  IOO  lengths  of  straight  iron  wiro 
o 


8a  ELECTRICAL  INSTRUMENT  MAKING  FOR  'AMATEURS. 

(best  soft  annealed)  No.  22  gauge,  say,  about  4$-  in. 
in  length  ;  fill  the  brass  tube  with  them  as  tight  as  you 
can  fit  them;  cut  them  all  to  the  same  length  (they  must 
protrude  a  little  beyond  the  tube).  Now  draw  out 
about  a  couple  cf  inches  of  the  iron  bundle,  and  wrap 
it  tightly  round  with  twine,  leaving  about  $  in.  free. 
Draw  more  out,. and  continue  v/rapping  until  you  have 
wrapped  to  within  \  in.  at  each  end  of  the  bundle. 
Tie  the  string,  and  withdraw  the  bundle  from  the  brass 
tube.  Melt  a  little  solder  in  a  ladle,  dip  the  ends  of  the 
iron  bundle  into  soldering  fluid  (zinc  dissolved  in  hydro- 
chloric acid),  and  then  at  once  into  the  melted  solder. 
Allow  the  bundle  to  cool ;  file  off  the  superfluous  colder, 
so  that  the  bundle  will  just  enter  freely  into  the  tube. 
It  should  appear  like  Fig.  27  when  the  string  has  been 
removed. 

§  50.  The  next  operation  is  to  make  a  good  stout 
paper  tube,  also  about  4^  in.  in  length,  into  which  the 
brass  tube  (Fig.  26)  can  slide  easily.  To  make  this,  put  a 
few  turns  of  soaped  writing  paper  round  the  tube  Fig.  26, 
then  roll  and  glue  .seven  turns  of  good,  stout  brown 
paper,  4$  in.  in  length,  round  this  writing  paper,  or  else 
it  will  be  difficult  to  draw  out  of  the  tube.  This  paper 
tube  (Fig.  28)  must  be  allowed  to  dry  thoroughly  while 
still  on  the  brass  tube  (Fig.  26).  When  quite  dry,  it  must 
be  slipped  off,  the  writing-paper  lining  drawn  out,  and 
then  it  must  be  soaked  for  a  few  minutes  in  melted 
paraffin  wax. 

§  51.  The  iron  bundle  should  also  be  allowed  to  stand 
in  melted  paraffin  v/ax  for  some  time,  and  then  stood 


THE  MEDICAL  COIL.  83 

up  to  dram  in  a  warm  place.  This  will  prevent  rusting. 
When  quite  cold,  all  superfluous  paraffin  having  been 
removed,  .a  strip  of  brown  paper,  \  in.  wide,  is  rolled 
round  one  extremity  of  the  iron  bundle,  until  it  is  of 
such  a  diameter  as  to  fit  tightly  into  the  paper  tube 
(Fig.  28).  This  paper  strip  must  be  cut  off  at  this  point, 
and  glued  tightly  round  the  end  of  the  iron  bundle. 
The  brass  tube  (Fig.  26)  is  then  slipped  over  the  iron 
bundle,  until  it  just  reaches  the  little  paper  collar  just 
made.  The  brass  tube1  and  bundle  together  are  pushed, 
button  end  first,  into  the  paper  tube  (Fig.  28),  and  when 
the  paper  collar  around- the  iron  bundle  is  just  about  to 
enter  the  paper  tube,  it  is  to  be  well  served  with,  hot 
glue  and  forced  into  the  tube.  The  whole  must  now  be 
allowed  to  dry  and  set  thoroughly. 

§  52.  Taking  one  of  the  3  in.  heads  (the  one  which 
has  not  any  nicks  in  the  sides)  we  bore  a  central  hole 
with  a  brace  and  centre  bit,  just  large  enough  for  the 
paper  tube  (Fig.  28),  with  its  iron  core,  to  fit  tightly 
(see  Fig.  29).  Putting  a  little  thin  good  hot  glue  round 
the  free  extremity  (the  end  opposite  that  at  which  the 
brass  enters),  we  push  it  into  the  hole  in  the  square 
head,  until  it  projects  about  i  in.  on  the  other  side. 
This  must  be  allowed  to  dry  thoroughly  before  pro- 
ceeding to  the  next  operation. 

§  53.  We  may  now  proceed  to  wind  the  primary  coil. 
To  this  end,  we  take  about  £  Ib.  of  No.  24  silk-coVered 
copper  wire,  and  wind  it  round  the  tube  (as  shown  at 
Fig.  33),  from  end  to  end  in  continuous  layers,  taking 
care  to  put  a  sheet  of  paraffined  paper  between  each 


84  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 

layer,  and  also  to  baste  each  layer  with  melted  paraffin 
wax- before  winding  on  another.  About  four  layers  will 
thus  be  got  on,  arid  an  even  number  of  layers  must  be 
aimed  at,  so  as  to  get  the  two  ends  of  the  wire  at  the 
same  extremity,  thus  rendering  it  easy  to  fasten  them 
under  the  binding  screws  A  A  (Fig.  33).  To  effect  this, 
before  screwing  down  the  said  screws,  the  ends  of  the 
copper  wire  are  stripped  of  their  covering  and  wound 
once  round  the  screw  of  the  binder.  Free  ends  of  wire, 
at  least  6  in.  in  length,  must  be  left  for  attachments, 
etc.  This  is  shown  at  Fig.  34. 

§  54.  This  primary  coil,  with  its  iron  core,  sliding 
brass  tube  regulator,  &c.,  may  now  be'  fastened  to-  the 
base  board  by  means  of  two  screws  from  underneath,  as 
shown  at  Fig.  32,  at  4  in.  from  one  end,  and  therefore 
8  in.  from  the  other.  One  of  the  free  ends  of  .the 
primary  wire  is  brought  to  one  of  the  binding  screws 
A,  while  the  other  connects  to  the  clapper,  B.  A  short 
piece  of  wire  connects  the  platinum  screw  pillar  c  to 
the  other  binding  screw,  which  is  not  visible,  as  it  is 
behind  the  platinum  pillar.  At  this  point  it  will  be  well 
to  try  the  working  of  the  primary  coil.  For  this  purpose 
couple  up  the  two  binding  screws  on  the  base  D9ard  with 
a  good  bichromate  cell.  Connect  the  two  binding 
screws  D  D  (Fig.  32)  with  the  two  brass  handles  intended 
for  use.  Screw  up  the  platinum  screw,  C,  until  the 
clapper,  B,  begins  to  vibrate.  Now  hold  the  handles  in 
your  hand.  As  long  as  the  brass  tube,  E,  is  entirely 
over  the  iron  core  little  or  no  sensation  is  perceptible. 
If  an  assistant  pull  out  the  tube,  little  by  little,  the 


THE  MEDICAL  COIL.  ^  85 

current  will  be  found  to  increase  in  strength  until  the 
regulator  tube  is  quite  out 

§  55.  The  secondary  coil  now  demands  our  attention. 
A  papef  tube,  precisely  similar  to  Fig.  28,  brit  of  such  a 
size  as  to  slide  easily  over  the  primary  coil  E  (Fig.  32), 
is  prepared,  and  paraffined.  This  must  be  cut  exactly 
the  length  of  the  coil  F,  leaving  the  knob  E  projecting. 
The  two  square  pieces  of  board  in  which  the  nicks  were 
cut  (Fig.  31)  must  have  central  holes  cut  in  them  to  take 
this  paper  tube,  and  then  be  glued,  one  at  each  end  of 
the  said  tube,  as  shown  at  Fig.  31.  Two  small  binding- 
screws  are  then  to  be  inserted  in  the  centre  of  the  upper 
edge  of  each  square.  A  bung  is  now  placed  in  each 
end  of  the  tube,  and  a  \  in.  iron  rod  pushed  through 
both,  to  serve  as  an  axle.  This  is  then  mounted  on  two 
standards,  as  shown  on  Fig.  35,  and  beginning  by 
attaching  one  end  of  the  uncovered  wire  to  the  binding- 
screw  A,  about  \  Ib.  No.  36  silk- covered  copper  wire  is 
now  coiled  pn,  being  most  diligent  in  avoiding  kinks, 
breaks,  or  flaws  of  every  description.  Each  layer  must 
be  paraffined  and  separated  from  its  neighbour  by 
paraffined  paper.  When  the  quantum  of  wire  has  been 
laid  on,  the  finishing  end  is  connected  to  the  binding 
screw  A1,  Fig.  35,  The  last  coil  should  be  covered  with 
paraffin  paper,  and  finally  covered  with  a  jacket  of  good 
silk  velvet.'  The  secondary  coil  is  then  complete,  and 
may  be  slid  in  its  place  over  the  primary  coil  (see  Fig. 
S<?)..  When  it  is  quite  over  the  primary,  the  secondary 
current  will  be  at  its  strongest,  if  the  metal  tube  regulator 
is  drawn  out;  it  v.'ill  be  weaker  as  the  metal  tube 


86  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEVRSi 


,* 

FIG.  29. 


FIG. 


W 


FIG.  31, 


FIG.  yt. 


THE  INDUCTION  COIL, 


s 


Fio.  33. 


FIG.  34. 
A 


\      \ 


flL 


FIG.  35. 


FIG.  36 


88  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMA  TEURS. 

regulator  is  more  and  more  inserted ;  or  may  be  even 
more  delicately  regulated  by  sliding  the  secondary  coil 
itself  more  or  less  over  the  primary.  The  secondary 
coil,  while  $he  primary  is-  being  excited  with  a  freshly 
made  pint  bichromate,  will  give  a  \  in.  spark,  when  the 
regulator  is  out,  and  the  secondary  coil  right  over  the 
primary.  This  will  pass  easily  through  a  dozen  persons. 

§  '56.  THE  INDUCTION  COIL. — If,  in  the  construction 
of  the  coil  described  in  the  last  eight  sections,  the 
following  modifications  be  made,  the  result  v/ill  be  a  coil 
Capable  of  giving  with  6  chromic  acid,  or  Bunsen  cells,  a 
good  inch  spark. 

'In  the  first  place,  a  brown  paper  tube  should  be  made, 
£  ins.  long,  f  in.  in  internal  diameter,  and  about  A  in. 
thick  by  rolling  and  gluing  the  brown  paper  as  recom- 
mended at  §  50,  round  any  cylindrical  rod  of  the  desired 
fliameter. r 

This  tube  should  be  treated  with  melted  paraffin  wax 
as  described  at  §  50,  and  then  filled  with  straight  iron 
wires,  No.  18  gauge,  as  tight  as  they  can  be  made  to  fit. 
(These  wires  will  not  require  soldering,  &c.,  as  they  are  to 
remain  permanently  in  the  paper  tube.)  The  wires 
should  protrude  about  £  in.  on  end  side  of  the  paper  tube. 

§  57  The  wooden  heads  should  be  only  two  in 
number,  and  4  ins.  square,  instead  of  3  ins.  as  directed 
at  §  49.  In  one  of  these  heads  (the  one  intended  to  be 
near  the  vibrating  hammer),  two  -h  in.  holes'are  drilled, 
close  to  the  central  hole  through  which  the  paper  tube 
is  passed.  The  two  heads  are  to  be  glued  on  to  the 
paper  tube,  precisely  as  recommended  for  the  secondary 


TItM  INDUCTION  COIL.  89 

coil  tube  at  §  49.  When  dry,  the  whole  affair,  wires 
and  all,  should  be  soaked  in  melted  paraffin  wax. 

It  should  be  then  set  up  between  standards,  and 
wound  carefully  with  four  layers  No.  18  double  silk 
covered  wire.  The  ends  of  this  wire  should  be  brought 
out,  at  the  hole  just  made  in  the  wooden  head.  When 
this,  the  primary  wire  has  been  well,  and  evenly  laid  on, 
it  should  be  well  basted  with  hot  melted  paraffin,  and 
then  surrounded  by  a  layer  of  paraffined  brown  paper, 
pulled  very  tight  and  smooth,  and  made  to  adhere  by 
means  of  melted  paraffin. 

Over  this  layer  of  paper  may  now  be  wound  the 
secondary  wire.  This  should  be  I  Ib.  of  No.  39,  in  one 
continuous  length :  and  should  be  tested  electrically  for 
continuity  before  laying  on.  The  whole  reel  of  wire 
should  be  soaked  in  melted  paraffin  wax,  and  allowed  to 
drain  while  still  hot,  before  being  wound  on  ;  each 
separate  layer  should  be  basted  with  hot  paraffin  wax 
before  being  covered  with  the  next  layer  of  paraffined 
paper.  The  ends  cf  the  secondary  should  be  attached 
to  the  binding  screws  marked  A  and  A1  at  Fig.  35,  §  55. 
The  coil  may  now  be  fitted  to  a  stand,  with  the  vibrating 
hammer  and  platinum  screw,  as  described  at  §  54;  but 
the  stand  chculd  take  the  fcrm  of  a  shallow  box  inverted, 
and  in  the  bottom  muctbs  fastened,  by  means  of  catches, 
a  Fizeau's  condenser  §  47,  and  the  hammer  pillar  and  the 
platinum  screw  pillar,  each  separately  connected  to  one 
of  the  projecting  lugs.  A  falsa  bottom  is  now  put  in  the 
stand,  to  hold  all  in  its  place.  The  condenser  is  essential  to 
Obtaining  a  good  long  and  dense  spark  Careful  insulation 


90  ELECTRICAL  INSTRUMENT  MAKING  FOX  AMATEURS. 

is  also  another  important,  if  not  the  most  important 
item,  in  a  successful  coil. 

§  58.  THE  MAGNETO-ELECTRIC  MACHINE. — This 
is  the  first  form  of  dynamo  which  the  inventive  genius 
of  Faraday  placed  before  the  scientific  world.  It  had 
its  origin  in  the  discovery  "that  a  conductor  moving 
before  the  poles  of  a  magnet,  in  such  a  direction  as  to 
cut  the  lines  of  force  of  the  said  magnet,  had  its 
electrical  condition  upset,  so  that  a  flow  of  electricity 
was  produced  within  it " ;  and  this  could  be  rendered 
evident  by  suitable  means.  Two  forms  of  magneto- 
electric  machines  will  be  described  :  firstly,  the  ordinary 
"  medical "  or  "  shocking  "  machine  ;  and  secondly,  a 
rather  more  scientific  instrument,  which  can  be  used" 
for  many  experiments,  for  which  the  former  wo\ild  be 
totally  useless. 

§  59.  THE  "SHOCKING"  MACHINE.— The  amateur 
will  need  a  pretty  powerful  horseshoe  magnet,  as  shown 
at  Fig.  37  M,  about  8  in.  long,  made  of  £  in.  steel. 
Each  limb  should  be  about  l£  in.  wide,  and  should  at 
the  polar  extremities  stand  about  $  in.  from  the  other. 
If  the  amateur  is  able  to  work  in  steel,  he  may  make 
these  magnets  himself,  and  magnetise  them  by  passing 
magnetising  coils  of  wire  round  each  limb,  and  sending 
a  powerful  current  of  electricity  through  the  coils,  until 
the  magnet  is  sufficiently  strong.  For  the  purpose 
required,  the  magnets  should  pull  easily  a  weight  of 
10  Ib. ;  but  it  will,  in  most  cases,  be  found  cheaper  to 
buy  these  magnets  ready  made.  The  armature  can  be 
readily  constructed  at  home.  It  consists  in  two  iron 


THE  "SHOCKING"  MACHINE.  51 

bobbins,  F  F*,  screwed  on  to  an  iron  yoke,  or  cross- 
piece,  Y ;  these  bobbins  should  be  turned  up  out  of  the 
very  best  soft  iron  circular  rod,  about  if  in.  diameter. 


.-*-- * 


FIG.  37. 

The  centres,  or  cores,  should  be  turned  down  to  about 
\  in.  in  diameter,  a  flange  about  rs  in.  in  thickness  being 
left  at  both  erds.  A  hole  must  be  drilled  up  the  centre 


92  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS,  j 

of  each  bobbin,  and  this  hole  must  be  tapped  to  receive 
an  iron  screw  about  I  in.  long,  A  in.  diameter.  These 
screws  serve  to  clamp  the  bobbins  to  the  yoke.  Owing 
to  the  weight  of  the  coiled  bobbins,  and  the  high 
speed  at  which  they  mUst  be  driven,  it  is  needful  that 
these  screws  should  be  strong  to  resist-  the  centrifugal 
tendency.  The  length  of  the  bobbins,  including  flanges, 
is  I  §  in.  If  the  operator  has  not  a  lathe,  he  may  make 
a  fair  substitute  for  the  turned  bobbins  by  putting  a 
screw-thread  on  the  top  and  bottom  of  two  pieces  of 
\  in.  iron  rod,  ig  in.  in  length,  and  screwing  thereto 
discs  of  soft  iron,  -rs  in.  in  thickness,  if  in.  in  diameter, 
having  holes  drilled  and  tapped  in  their  centres  to  take 
the  extremities  of  the  rods.  The  yoke  Y  has  in  its 
centre  a  I  in.  hole,  which  must  be  carefully  bushed  with 
some  good,  hard  insulator,  such  as  ebonite,  ivory,  or 
boxwood,  soaked  in  paraffin  wax. 

§  6cx  The  shaft  or  spindle  on  which  the  armature 
revolves  constitutes  the  distinguishing  feature  of  this 
particular  form  of  magneto  machine.  It  must  be  com- 
pact, easy  to  construct,  strong,  and  well  insulated  in'  its 
two  halves.  It  must  also  have  some  device  whereby 
contact  can  be  made  and  broken  two  or  three  times 
during  each  revolution.  To  this  end  an  iron  rod,  about 
5  in.  long  and  £  in.  in  diameter,  is  turned  down  at  both 
ends  for  a  length  of  \  in.,  to  about  A  in.  in  diameter. 
This  is  to  produce  a  shoulder  at  each/  end  for  the 
spindle  to  rest  in  its  bearings.  These  thinner  ends  are 
shown  at  A  and  A'.  Another,  A  in.,  is  taken  off  at  one, 
end  for  about  \  in.,  say,  at  the  end  A'.  Then  a  screw-! 


THE  "SHOCKING"  MACHINE.  93 

thread  of  about  \  in.  in  length  is  run  on  at  D'.  The 
rod  is  then  cut  in  two  at  D,  and  a  length  of  about 
\  in.  is  turned  down  to  about  i  in.  in  diameter.  A 
screw  must  be  formed  oh  this  thinner  portion,  as  shown 
at  D.  The  portion  B'  has  then  a  hole  about  |  in.  in 
depth  drilled  in  it,  and  about  -is  in.  in  diameter.  A 
female  screw  must  be  cut  in  this,  and  the.  hole  bushed 
with  ivory  or  ebonite,  as  shown  at  C.  Great  care  must 
be  taken  in  tapping  this  to  receive  the  screwed  end 
of  D,  that  the  two  halves  of  the  divided  rod  are  perfectly 
insulated  from  one  another.  To  this  end  an  ebonite 
washer  is  placed  at  w.  It  is  well  to  test  for  insulation 
by  inserting  the  spindle  at  this  point,  in  the  circuit 
between  a  galvanometer  and  a  Battery.  If  any  current 
passes  so  as  to  produce  a  deflection,  the  -hole  C  must  be 
cleaned  out  and  replugged,  until  perfect  insulation  has 
been  effected.  Two  small  holes,  to  take  short  screws 
about  tk  in.  diameter  by  J  in.  in  length,  must  be  drilled 
and  tapped,  near  B  and  B'.  These  are  intended  to 
make  connection  with  the  two  ends  of  the  wire  coming 
from  the  bobbins,  as  shown  at  Q.  The  next  operation 
is  to  turn  up  a  small  brass  pulley,  about  A  in.  in 
thickness,  by  about  $  in.  in  diameter,  which  must  have 
a  female  screw  put  in  it  to  fit  over  D',  as  illustrated 
at  P.  This  pulley  serves  at  one  time  to  clamp  the  yoke 
Y  in  its  place  on  the  spindle,  and  to  communicate  tho 
motion  from  the  little  driving-v/heel  to  the  armature 
spindle.  All  that  now  remains  to  be  done  to  the 
spindle  is  to  file  about  J  in.  of  its  length,  just  beyond 
the  screw-thread  D',  into  a  triangular  form,  like  the 


94  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS 

Grecian  letter  A,  as  shown  at  E'  and  E.  The  length  of 
the  finished  spindle  should  be  3^  in. 

§  61.  A  brass  frame,  of  the  form  "and  dimensions 
figured  at  Fig.  38  A,  must  now  be  provided.  In 
substance  it  should  be  £  in.  wide  by  \  in.  deep.  At  the 
point  d  the  frame  expands  both  above  and  below,  so  as 
to  form -a  lug,  projecting  about  \  in.  on  either  side, 
against  which  can  be  clamped  the  magnet,  M  (Fig.  37). 
The  amateur  should  construct  a  pattern  of  this  frame 
in  wood  a  trifle  larger  than  it  is  intended  to  be  (to  allow 
for  shrinkage),  and  after  having  carefully  smoothed  and 
bevelled  the  edges,  send  it  to  the  brass  founder's  to  get  a 
similar  one  cast  in  brass.  When  cast,  the  frame  will 
require  careful  cleaning  and  trueing  up  with  a  file. 
Holes  to  take  -ft  in.  diameter  screws  must  then  be 
drilled  and  tapped  at  c  c.  These  serve  to  fasten  the 
frame  into  its  box.  At  B  and  B'  rather  larger  holes 
must  be  bored,  the  former  being  carefully  bushed  with 
ivory  or  hard  ebonite,  the  latter  fitted  with  a  metal 
screw,  through  both  of  which  a  hole  is  put, sufficiently 
large  to  serve  as  bearings  for  the  end,  A  and  A',  of  the 
spindle  (see  Fig.  37-.  AA*).  At  e  and  e\  precisely 
opposite  one  another,  are  drilled  two  A  holes,  which 
serve  as  bearings  for  the  driving-wheel.  At  /,  a  small 
hole  about  \  in.  and  i  in.  deep,  is  drilled  and  tapped,  to 
receive  a  small  screw.  This  js  intended  to  receive  the 
small  contact  spring  R  (Fig.  37.) 

§  62.  The  next  tiling  needed  is  a  small  brass  driving- 
wheel  about  4  in.  in  diameter  and  J  in.  thick,  with  a 
grooye  cut  in  the  periphery,  to  take  a  gut  band.  The 


THE  "SHOCKING     MACHI  93 

tyro  need  not  make  a  pattern  for  this,  for  nearly  e^ery 
toy-engine  shop  keeps  brass  flywheels  of  about  this  size. 
If  it  have  not  a  groove  when  bought,  one  can  easily  be 

U—  -  £//---* 
f 4% */^\       :;' 


;; 

i.            '  e 
.-.  c 
e' 

rl 

'    '  Ml     -  =^*  "  "         L "V 


"TV 


FIG.  38. 

put  in  it  on  the  lathe.  This  wheel  fe  shown  at  Fig.  38, 
G,  mounted  on  an  iron  shaft,  3  in.  long,  H.  This  fly- 
wheel may  be  keyed  or  brazed  to  the  shaft  Just 
beyond  the  fly-wheel  at  I,  the  shaft  is  turned  down 


96  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 

so  as  just  to  enter  freely  into  the  holes  e  and  ^. 
At  the  extremity,  G,  a  screw  thread  is  put  on  the 
projecting  end  of  the  rod,  to  take  the  female  screw  of 
the  driving  handle,  shown  at  J.  At  the  opposite 
extremity,  H,  the  shaft  is  drilled  and  tapped  to  take  a 
rather  large-headed  screw,  which  serves  to  prevent  the 
shaft  from  riding  to  and  fro. 

§  63.  The  operator  may  now  proceed  to  wind  the 
bobbins.  For  this  purpose  he  will  need  about  I  Ib.  of 
No.  26  or  28  double  silk-covered  wire.  Having  ascer- 
tained, by  testing  with  a  battery  and  galvanometer,  that 
the  wire  is  continuous,  it  will  be  well  to  soak  the  coil  of 
wire  for  a  few  minutes  in  hot  melted  paraffin  wax, 
allowing  the  paraffin  afterwards  to  drain  off,  by  suspend- 
ing the  coil  for  a  few  minutes  in  a  rather  warm  place.  The 
iron  bobbins,  F  F'  (Fig.  37),  are  then  wrapped  round  with 
one  layer  of  brown  paper,  which  is  fastened  down  to  the 
cores  with  good  glue.  Two  semicircles,  with  hole  cut  in 
the  interior  to  fit  the  iron  cores,  are  also  glued  inside 
the  flanges,  so  that  the  whole  of  the  inside  of  the 
bobbins  is  entirely  covered  with  brown  paper.  When 
the  glue  is  quite  </>j,  the  bobbins  maybe  warmed,  and 
plunged  for  a  few  seconds  in  hot  melted  paraffin.  Each 
bobbin  is  then  to  be  wound,  in  the  same  direction^  with 
the  wire  above  specified,  until  the  bobbin  is  filled  to  the 
top  of  the  flange.  To  prevent  unwinding  the  finished 
ends  may  be  tied  down  to  the  bobbin  with  a  bit  of  silk, 
leaving,  however,  about  2  in.  of  the  wire  free  for  attach- 
ment  to  the  spindle.  The  commencing  ends  of  the 
wire  must  also  be  allowed  to  protrude  beyond  the  flange 


THE  "SHOCKING"  MACHINE.  57 

for  2  in.  or  3  in.  fat  connection.  The  two  bobbins  are 
then  screwed  firmly  to  the  yoke,  the  yoke  afterwards 
being  put  on  the  spindle.  When  this  has  been  done  the 
four  ends  of  the  wires  are  joined,  as  shown  at  Q  (Fig.  37)  ; 
that  is  to  say,  the  two  lower  ends  (after  their  covering 
has  been  removed),  are  twisted  together,  soldered,  and 
rubbed  over  with  Prout's  elastic  glue,  to  insure  insula- 
tion ;  while  the  two  top  ends  are  screwed  each  one 
down  to  the  shaft,  one  at  B,  and"  the  other  at  B',  the 
covering  having,  been  previously  removed  from  the  wire 
at  these  points  to.  insure  perfect  electric  contact  with  the 
two  halves  of  the  shaft.  In  order  that  the  machine 
may  work,  the  relation  between  the  winding  and  connec- 
tions of  the  lower  ends  of  the  bobbin  wires  (as  seen 
from  the  yoke  end),  must  be  like  this — CO.  A  thin 
ebonite  washer  is  now  placed  over  the  yoke,  and  the 
little  pulley,  P,  screwed  tightly  in  its  place.  This  must 
be  screwed  down  very  firmly,  as  upon  this  depends  the 
stability  of  the  armature.  The  armature  is  noc 
finished,  as  far  as  actual  work  goes,  but  the  bobolns  may 
be  covered  with  any  pretty-coloured  silk  velvet,  if 
appearance  be  studied. 

§  64.  The  brass  frame  is  now  clamped  by  means  of 
a  stout  brass  cross-piece  K  (Fig.  38)  to  the  magnet;  and 
if  necessary  a  second  cross-piece  (also  of  brass)  is  screwed 
to  the  frame  at  L.  The  back  end  of  the  armature 
spindle  (the  end  farthest  from  the  pulley)  is  passed  into 
the  larger  hole  B,  of  the  frame  A.  This  will  allow  the 
other  end  of  the  spindle  being  inserted  into  the  opposite 
hole  B,  without  strain.  When  the  spindle  is  in  its  place 


$8    ELECTRICAL  INSTRUMENT  MAKING  FOR  AMA  TEURS. 

the  hollow  screw  is  place  in  B1,  and  screwed  home,  until 
the  armature  spindle  can  just  turn  freely,  without  too 
much  play,  before  the  poles  of  the  magnet  In  like 
manner  the  driving-wheel  G  is  put  in  its  place  by  passing 
the  long  end  of  spindle  through  the  holes  e  and  e,  and 
then  fixing  it  in  position  by  means  of  the  larger  headed 
screw  already  mentioned  at  the  end  of  §  62. 

The  small  spring  R  is  then  screwed  down  to  the  frame 
at/!  It  mustjusf  rest  on  the  projecting  corners  of  the 
triangle  E1,  during  rotation,  and  just  clear  the  flattened 
portions.  On  careful  attention  to  this  point  a  great 
deal  of  the  efficiency  of  the  machine  depends.  The 
shock  is  felt,  not  while  the  current  is  continuous,  but  at 
the  instants  of  breaking  and  making  contact.  The 
instrument  can  now  be  placed  in  any  suitable  box,  which 
must  be  sufficiently  long  to  allow  the  armature  to  rotate 
without  striking  against  the  ends,  and  just  a  trifle — say 
$  in. — wider  than  the  frame.  The  frame  is  attached  to 
the  box  by  means  of  screws  which  pass  through  the 
box  into  the  holes  c  and  c1  of  the  frame.  The  screw 
at  c  "should  be  hollow,  so  as  to  take  a  small  pin,  or 
metal  hook,  which  is  intended  to  make  connection 
with  the  metallic  cords  and  handles,  that  are  used  for 
giving  shocks.  Another  similar  screw  is  put  in  the  side 
of  the  box  almost  opposite  the  armature,  and  to  this 
screw,  in  the  inside  of  the  box,,  is  fastened  a  rather  stiff 
brass  spring  about  \  in.  wide,  bent  into\he  shape  of  L, 
the  longer  end  of  which  must  press  firmly  against  the  end 
of  the  armature  spindle  which  projects  through  the  hole  B 
Fig.  38  .  This  latter  screw  forms  the  other  terminal  of  the 


THE  UNI-DIRECTION  CURRENT  MACHINE.          99 

machine,  the  other  cord,  etc.,  being  affixed  thereto.  It 
is  usual,  though  not  essential,  to  place  a  soft  iron  keeper 
at  the  back  of  the  magnet's  poles,  which  may  be  partially 
or  wholly  removed  at  will  This  enables  the  operator 
to  regulate  somewhat  the  strength  of  the  shocks  by 
increasing  the  inductive  effect  of  the  magnet  on  the 
armature,  consequent  on  the  removal  of  keeper  from  near 
its  poles.  There  must  also  be  a  hole  in  the  box,  to 
allow  of  the  insertion  of  the  driving- handle,  J. 

§  65.  THE  UNI-DIRECTION  CURRENT  MACHINE. — 
rAt  §  59  we  studied  the  construction  of  the  magneto 
specially  designed  for  physiological  effects.  Here  we 
shall  direct  our  attention  to  one  that  may  be  used  for 
heating,  lighting,  chemical,  and  electro- magnetic  experi- 
ments generally. 

The  first  thing  to  be  procured  or  made,  as  described 
at  §  59,  is  a  horse-shoe  magnet,  having  a  clear  space  of 
I  in.  between  the  poles,  8  in.  long,  \  in.  thick,  and  it  in. 
wide.  This  should  be  capable  of  lifting  at  least  IO  Ib. 

An  armature,  or  iron  core,  3  in.  long,  ^  in.  diameter,1 
with  a  channel  all  round  its  length,  I  in.  wide,  |  in.  deep 
at  the  sides,  j  in.  deep  at  the  ends,  as  shown  at  Fig.  39,  must 
next  be  made.  It  may  be  cast  in  good  malleable  iron, 
and  carefully  annealed  ;  but  it  will  give  better  results  if 
cut  out  of  a  soft  piece  of  circular  wrought-iron  rod,  which, 
with  a  good  file  and  a  little  patience,  may  be  filed  up  to 
the  desired  shape.  Should,  however,  a  casting  be  pre- 
ferred, the  pattern  for  the  purpose  may  be  cut  out  of  a 
common  broom  stick,  nicely  sand-papered  to  the  desired 
size,  the  channel  being  cut  out  with  a  sharp  chisel. 


loo  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 

Whichever  plan  may  be  adopted,  the  armature,  pre- 
vious to  windiftg>  must  be  fitted  with  two  brass  or  gun- 
metal  heads,  wherein  the  spindle  is  affixed.  For  this 
purpose,  the  amateur  who  has  no  lathe  must  strike  two 
circles  }g  in.  diameter  on  a  piece  of  hard  sheet  brass,  J  in. 
thick,  great  care  being  taken  to  get  the  circles  true,  and 
the  centres  distinctly  marked.  These  circles  can  be  cut 
out  roughly  with  a  hack-saw,  and  finished  up  carefully 
to  the  line  with  a  file.  A  perfectly  central  hole  is  to  be 
drilled  in  each  disc  with  an  Archimedean  drill,  to  take 
circular  iron  rod  A  in.  diameter.  Two  smaller  holes, 


FIG.  39. 

one  on  each  side  of  the  central  aperture,  are  also  drilled 
through  the  brass  circlets.  These  are  to  take  the  screws 
which  hold  the  heads  on  to  the  armature.  Holes  cor- 
responding exactly  to  these  in  position,  must  now  be 
drilled  in  the  two  extremities  of  the  armature,  the 
greatest  care  being  taken  that  the  heads  are  fitted  truly, 
so  that  the  armature  may  run  without  wobbling.  These 
holes  in  the  armature  must  be  tapped  so  as  to  take  the 
screws. 

On  the  brass  circlet  which  will  afterwards  be  placed 
nearer  to  the  commutator,  must  be  drilled  yet  two  more 
holes,  through  which  the  ends  of  the  wire,  with  which 


THE  VNI-DIRECTION  CURRENT  MACHINE.         10! 

the  armature  will  be  wound,  must  protrude.  When 
the  armature  and  heads  have  thus  been  fitted,  two 
pieces  of  the  ±V  in.  iron  rod  before-mentioned  should  be 
cut,  one  for  the  back  and  the  other  for  the  front  or  com- 
mutator end,  the  former  being  about  li  in.  long,  the 
latter  2  in.  A  thread  must  be  put  on  the  end  of  each 
of  these  rods  where  they  enter  the  brass  circlets,  and 
after  being  screwed  in,  the  end  must  be  burred  over  by 
hammering,  so  as  to  prevent  the  rods  (which  serve  as 
the  spindle  for  the  armature)  from  working  loose  during 
rotation./ 


§  66.  A  small  piece  of  boxwood  about  I  in.  long  is 
now  to  be  cut  or  turned  into  a  perfect  cylinder  about 
}  in.  diameter.  A  hole  •&  in.  diameter,  is  put  centrally 
through  this,  so  that  it  can  be  made  to  fit  tightly  on  the 
spindle.  A  piece  of  brass  tubing,  about  TV  in.  thick,  i 
in.  long,  and  I  in.  in  internal  diameter,  is  now  driven  on 
to  the  boxwood  cylinder.  Two  short  brass  screws,  not 
reaching  to  the  iron  spindle,  must  now  be  procured,  and, 
holes  having  been  drilled  at  two  opposite  points  in  the 
diameter  of  the  ring,  and  countersunk,  the  screws  are  in- 
serted and  tightly  screwed  down  so  as  to  hold  the  ring 
in  its  place.  The  brass  tube  or  ring  is  now  cut  into  two 


102  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 

halves  by  giving  two  fine  saw  cuts  across  the  ring  at 
two  points  equi-distant  from  the  screws  just  inserted. 
This  completes  the  commutator,  which  is  now  ready 
to  be  slipped  on  the  longest  end  of  the  spindle  when 
required. 

§  67.  The  next  operation  is  to  wind  the  armature ; 
and  here  the  amateur  must  use  his  own  discretion  as  to 
whether  he  will  wind  it  for  a  large  current  of  low  E.M.F., 
or  for  a  high  E.M.F.,  but  little  current  If  he  desires 
to  have  both  he  will  do  well  to  make  two  armatures 
precisely  as  described,  and  wind  the  one  with  coarse 
wire,  say  No.  14,  for  large  currents,  and  the  other  with 
No.  22,  for  smaller  currents  of  higher  E.M.F.  Previous 
to  winding,  however,  the  armature  must  be  taped,  as  it 
is  technically  called,  to  insure  perfect  insulation. 

To  this  end  the  brass  heads,  with  the  spindle,  etc.,  are 
removed,  and  a  piece  of  silk  ribbon,  i  in.  wide,  glued 
right  round  the  central  portion  of  the  channel  of  the 
armature  so  as  to  entirely  cover  the  iron.  In  like 
manner,  the  sides  of  the  channel  are  also  covered  with 
silk.  When  the  glue  is  quite  dry,  the  armature  may  be 
immersed  bodily  in  melted  paraffin  wax,  so  as  to  satu- 
rate the  silk  thoroughly  with  paraffin.  It  should  then 
be  taken  out,  allowed  to  drain,  and  set 

N§  68.  The  armature  may  now  be  wound.  The 
operator  takes  the  armature  in  his  left  hand,  with  that 
end  which  is  to  be  against  the  commutator  nearest  to 
him.  Holding  in  his  left  hand  also,  about  6  in.  of  spare 
end  of  the  wire  with  which  he  intends  to  wind  the 
armature,  he  grasps  the  reel  or  hank  in  his  right,  and. 


THE  UN1-DIRECT10N  CURRENT  MACHINE.         103 

winding  always  in  the  same  direction,  fills  up  the  channel 
with  the  wire,  taking  care  to  wind  tightly,  smoothly,  and 
evenly,  and  being  most  careful  not  to  abrade  the  covering 
of  the  wire,  so  as  to  produce  leakage.  About  3  oz.  can 
be  laid  on  without  extending  beyond  the  sides  of  the 
channel.  (See  Fig.  40'.) 

It  will  be  well,  before  finishing  the  armature  to  test 
the  wire  for  insulation.  This  can  be  done  by  connecting 
one  end  of  the  wire  to  one  pole  of  a  single-cell  chromic 
acid  battery,  the  other  pole  of  which  is  connected  to 
one  terminal  of  a  galvanometer.  A  short  length  of  wire 
is  attached  to  the  other  terminal  of  the  galvanometer. 
If  with  this  wire  the  iron  part  of  the  armature  be 
touched  no  deflection  will  ensue  if  the  insulation  between 
the  coiled  wire  on  the  armature  and  the  iron  body  of 
the  armature  is  good:  but  if  it  be  defective,  the  needle 
will  swing  round  and  indicate  the  fact.  It  is  needless 
to  point  out  that  the  iron  of  the  armature  should  be 
sand-papered  at  the  spot  where  it  is  to  be  touched  with 
the  wire,  as  otherwise  the  paraffin  wax  adhering  to  it 
might  falsify  the  results  of  this  trial. 

Both  the  beginning  and  ending  portions  of  the  coiled 
wire  must  be  brought  out  at  the  commutator  end  of  the 
armature.  The  heads  of  the  armature  can  now  be 
permanently  screwed  on,  the  ends  of  the  coiled  wire 
pushed  through  the  two  holes  left  in  the  head,  the  wire 
pulled  very  tight,  the  holes  bushed  with  melted  gutta- 
percha  to  prevent  any  chance  contact  of  the  wires  with 
the  brass  head,  then  the  commutator  pushed  on  to  the 
spindle  (a  drop  of  while  hard  varnish  having  been  pre- 


104  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS] 

viously  applied  thereto  to  insure  adhesion)  ;  and  lastly 
the  two  projecting  ends  of  the  wire,  cleaned  from  their 
covering,  and  soldered,  one  to  each  brass  cheek  of  the 
commutator,  as  shown  in  the  cut  at  Fig.  40,  which  also 
shows  the  position  the  slits  on  the  commutator  should 
occupy  with  relation  to  the  channel  of  the  armature. 
N.B. — Only  one  slit  is  shown,  as  also  only  one  wire, 
since  the  other  is  hidden  by  the  spindle.) 

§  69.  The  magnet  should  now  be  mounted  on  a  stained 
wood  or  mahogany  base  board,  about  14  in.  long,  by  8  in. 
wide,  and  I  in.  thick.  Two  pieces  of  well-seasoned  wood 
(deal  will  do)  i£  in.  in  square  section  and  about  6  in. 
long  are  to  be  glued  one  each  side  of  the  centre  of  the 
base  board,  so  that  the  magnet  can  lie  with  each  pole 
supported  on  one  of  these  strips,  leaving  a  clear  place 
for  the  armature  to  lie  between  without  touching  any- 
thing. 

A  piece  of  square  brass  rod  1  in.  in  "section  and  4!  in. 
long  is  next  procured.  A  i  in.  hole  is  drilled  at  each 
extremity  to  take  a  long  screw.  At  the  centre,  another 
piece  of  similar  brass,  $  in.  square,  but**only  j  in.  thick, 
is  soldered,  and  through  this  is  drilled  a  iV  hole  to 
serve  as  a  bearing  for  the, back  spindle  of  the  armature. 
This  piece  of  brass  serves  a  double  purpose.  It  acts  as  a 
strap  td  hold  the  magnet  in  its  place  when  screwed  down 
by  the  two  side  screws  ;  it  also  serves  to  carry  the  bear- 
ing. The  exact  distance  of  this  brass  strap,  from  the 
poles  of  the  magnets,  depends  on  the  length  of  the  arma- 
ture ;  it  should  be  so  placed  that  when  the  spindle  is  in 
the  bearing  up  to  the  brass  head,  the  front  head  should 


THE  UNI-DIRECriON  CURRENT  MACHINE.          10$ 

just  be  flush  with  the  poles  of  the  magnet.  The 
appearance  of  the  magnet,  supported  on  the  two  wooden 
blocks  and  held  down  with  the  straps,  is  well  shown 
at  Fig.  41. 

For  the  front  bearing,  a  piece  of  stout  sheet  brass 
about  |  in.  in  substance  is  cut  into  the  shape  of  an  angle 
about  li  in.  wide  at  the  base,  2  in.  high,  rounded  at  the 
apex.  The  base  of  this  triangle  is  bent  at  right  angles 
to  the  rest  to  the  depth  of  about  1  in.,  in  order  to  form  a 


FIG.  41. 

foot  Two  holes  must  be  drilled  into  this  reversed 
portion,  into  which  are  passed  two  screws  to  attach  it  to 
the  base  board.  The  armature  having  been  placed  in 
the  back  bearing,  is  supported  by  a  cork  and  side 
wedges,  so  that  it  stands  equi-distant  from  i  either 
pole  of  the  magnet,  touching  at  no  part.  The  front 
bearing  is  then  rested  on  the  base  board,  and  pushed 
against  the  front  spindle.  A  mark  is  made  where,  it 
touches,  and  a  A  hole  drilled  through  the  bearing  at  this 
sj)ot  This  must  be  cleared  out,  so  that  the  armature  can 


106  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 

rotate  freely  between  the  poles  of  the  magnet,  supported 
on  the  back  and  front  bearing,  without  any  strain,  twist 
or  bind. 

A  keyway  must  be  filed  on  the  back  spindle,  and  a 
small  $  in.  grooved  pulley  keyed  thereon. 

§  70.  In  order  to  collect  the  electricity  set  up  by  the 
rotation  of  the  armature,  two  "  brushes "  are  needed 
These  consist  in  two  strips  of  sheet  copper,  about  S1o  in. 
thick,  hammered  until  quite  springy,  and  then  cut  about 
\  in.  wide,  by  i\  in.  long.  At  about  \  in.  from  one  end 
of  each  strip  is  punched  a  circular  hole,  wide  enough  to 
take  the  tang  of  a  binding  screw.  Two  little  blocks  of 
boxwood,  about  \  in.  in  square  section,  one  being  f  in. 
high,  the  other  about  i\  in.  in  height,  are  now  to  be  glued 
on  to  the  base  board,  one  on  each  side  of  the  commutator 
(§  66)  at  a  distance  of  about  if  in.  from  it  To  insure 
rigidity  and  strength,  these  two  blocks  should  be  made 
longer  than  they  are  intended  to  stand  above  the  level  of 
the  base  board,  a  corresponding  square  hole  being  cut 
into  the  base  board  with  a  chisel.  On  gluing  these 
blocks  in  their  places,  there  will  be  no  fear  of  there 
becoming  detached. 

When  quite  dry,  a  small  hole  (a  little  less  than  the 
tang  of  the  binding  screws  which  are  to  be  used)  is 
drilled  centrally  and  perpendicularly  in  these  blocks. 
Each  copper  strip  or  "brush"  is  now  bent,  so  that  when 
lying  on  its  block  it  j**st  touches  the  commutator  ring ; 
that  on  the  higher  block  resting  on  the  commutator 
above,  while  the  one  on  the  lower  block  can  be  just 
caused  to  touch  the  commutator  underneath  Th,2 


THE  UNI-DIRECTION  CURRENT  MACHINE.          107 

binding  screws  are  then  inserted  in  the  holes  through 
the  brushes  into  those  in  the  blocks,  and  then  screwed 
down  tightly,  so  as  to  hold  the  brushes  firmly  in  their 
place,  pressing  lightly  against  the  commutator  above  and 
below.  These  two  binding  screws,  as  shown  at  Fig.  42, 
serve  as  the/^/?j  or  terminals  of  the  machine,  and  to 
these  the 'wires,  etc.,  may  be  attached  to  lead  the  current 
where  desired. 


FIG.  42. 

§  71.  The  only  thing  now  needed  is  a  bracket  and 
hand-wheel,  wherewith  to  drive  the  armature.  The 
simplest,  and  at  the  same  time  the  strongest  mode  of 
affixing  these,  is  to  have  a  A-shaped  bracket  cast  in  iron 
at  the  nearest  foundry.  This  should  be  9  in.  in  height, 
and  should  stride  over  the  magnet)*  clearing  it  on  each 
side  by  about  I  in.  Holes  must  be  drilled  in  the  feet  to 


io8  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS! 

screw  it  to  the  base  board,  and  at  the  top  a  hole  must  be 
put  through  it  to  take  a  bolt  and  nut.  A  12  in.  wheel, 
either  of  iron  or  wood,  and  furnished  with  a  wooden 
handle,  is  put  on  this  bolt,  and  a  back-nut  screwed  on  to 
keep  it  against  the  bracket.  A  gut  or  leather  band 
connects  the  hand-wheel  to  the  pulley  at  the  rear  of  the 
armature.  The  complete  machine  is  shown  at  Fig.  42. 

With  this  machine  a  variety  of  instructive  experi- 
ments can  be  performed.  Small  lamps  can  be  lit,  an 
arc  light  can  be  shown,  coils  worked,  water  decomposed, 
etc.  The  direction  of  the  current  can  be  changed  by 
changing  the  direction  of  rotation  ;  and  by  sending  the 
current  from  a  battery  into  the  armature  the  machine 
gives  a  proof  of  its  reversibility  by  running  as  a 
motor. 

$  72.  THE  DYNAMO.— In  another  work*  the  author 
has  gone  pretty  fully  into  the  theory  and  practice  of  the 
construction  of  dynamos  of  the  Siemens'  type.  It  is 
proposed  here  to  give  a  general  outline  of  the  mode  of 
calculating  the  winding  of  dynamos  of  the  ring  arma- 
ture type,  along  with  such  instructions  as  will  enable  the 
amateur  to  decide  upon  the  size  of  the  armature  and 
field  magnets  required  to  produce  a  given  effect. 
Machines  of  the  "ring"  class  may  be  conveniently 
constructed  either  of  the  A  Gramme  form,  as  shown  in 
the  annexed  Fig.  43,  or  of  the  form  Fig.  44,  now  generally 
known  as  the  "  Manchester "  type,  due  to  Mather, 
Hopkinson,  and  Platt.  Wooden  patterns  should  be 
made  of  the  fields,  from  which  good  malleable  iron  cast- 
*  The  Dynamo  :  How  made  and  how  used. 


THE  DYNAMO. 


109 


ings  can  be  obtained  from  any  founder  who  knows  his 
work.  The  castings  must  be  soft,  or  else  quite  40  per 
cent,  of  the  efficiency  of  the  machine  will  be  lost 

The  ring  (which  should  be  of  the  toothed  or  Pacinotti 
form  shown  at  Fig.  44,  may  be  also  cast,  but  will  be  better 
in  every  respect  if  built  up  of  laminations  in  sheet  iron, 
of  the  same  shape  as  shown,  and  strung  together  by 
pins  running  through  every  alternate  tooth,  which  pins, 
if  screw-headed,  serve  to  bolt  the  armature  firmly  to 


FIG.  43. 

the  brass  star-wheel,  or  "  spider,"  by  means  of  which  it  is 
affixed  to  the  shaft.  As  these  laminations  or  "  punch- 
ings  "  can  be  had  ready  made  in  many  convenient  sizes 
of  the  makers  of  electrical  instruments,  the  manufacture 
of  such  is  not  advised  here,  as  a  punching  tool  costs  from 
.£3  to  £4.  to  make  ;  whereas  the  punchings  can  be  had 
for  a  few  shillings  per  gross. 

The   amateur,  having   decided  on  which  patterns  he 
intends  to  employ  for  the  fields,  has  next  to  consider  the 


I  to  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 

purpose  to  which  he  intends  to  put  the  machine  when 
complete,  and  the  power  he  has  to  drive  it.  It  is  really 
the  former  of  these  data  which  will  decide  the  size 
of  the  armature,  and,  consequently,  of  the  machine 
itself. 

First  then,  as  to  power.  The  best  modern  dynamos 
return  about  95  per  cent,  of  the  energy  spent  upon 
them,  as  electricity.  Theoretically,  I  h.p.  is  equal 
to  746  watts.  The  watt  is  I  arnp^re  multiplied  by 
I  volt,  so  that  we  are  at  liberty  to  take  our  "  watts  "  out 

I 


PIG.  44. 

of  our  dynamos  as  we  please.  We  can  have,  for 
instance,  a  low  tension  machine  of  only  2  volts  E.M.F., 
but  giving  a  current  of  373  amperes,  or  we  may  have  a 
small  current, of  2  amperes  only,  with  an  E.M.F.  of  373 
volts ;  in  either  case  746  watts  would  be  generated  by 
the  dynamo,  and  rather  more  than  I  h.p.  needed  for 
its  evolution. 

The  next  question  to  be  considered  is,  what  number  of 
volts  is  the  machine  to  give  ?  Having  decided  this  point, 
the  length  of  the  wire  to  be  employed  is  immediately  found  j 


THE  DYNAMO.  UI 

since  large,  well-constructed  machines,  give  on  an  average 
I  volt  for  each  yard  of  copper  coil  in  the  armature. 
Therefore,  so  many  volts  as  are  desired  as  the  E.M.F.  of 
the  finished  machine,  so  many  yards  of  wire  on  the 
armature.  If  the  machine  is  a  small  one,  i.e.,  does  not 
greatly  exceed  2  cwt.  in  weight,  it  will  be  well  to  allow 
more  than  a  yard  per  volt,  as  small  machines  are  not  so 
efficient  as  large  ones.  In  calculating  the  voltage,  the 
desired  current  must  also  be  considered,  and  allowance 
made  both  for  the  internal  and  external  resistances,  as 
also  for  the  amount  of  current  required  to  keep  up  the 
requiske  intensity  of  the  magnetic  field.  Remembering 
Ohm's  law  that  the  current  will  be  equal  to  the  E.M.F. 
in  volts,  divided  by  the  R  in  ohms,  we  can  easily  (know- 
ing what  resistance  we  are  going  to  have  in  the  outer 
circuit)  calculate  from  a  table  of  wire  resistances,  what 
gauge  copper  wire  we  may  use  on  the  armature,  so  that 
the  length  desired  to  produce  the  desired  number  ol 
volts  shall  not  exceed  in  resistance  the  margin  left  from 
the  outer  circuit,  to  allow  of  the  passage  of  the  desired 
number  of  amperes.  In  making  this  calculation,  it  must 
always  be  borne  in  mind,  that  even  in  the  best  made 
shunt  dynamos,  it  is  usual  to  allow  4  to  5  per  cent,  of  the 
current  to  be  shunted  off  through  the  field  magnet  coils. 
The  annexed  table  of  sizes,  weights,  lengths,  and  resis- 
tances of  the  usual  covered  copper  wire  found  in 
commerce,  will  enable  the  amateur  to  calculate  at  once 
the  desired  gauge  and  length  of  wire  to  be  used  to 
furnish  a  given  E.M.F.  and  current : — 

§73.      Table  showing  relative  resistances/ weights 


ii2    ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 

and    length,    of   different    gauges   of    covered    copper 
wire. 


B.  W.  G. 

Decimal 
of  an 
inch. 

Equiva- 
lent in 
m/m. 

Yards  to  the  lb., 
approximate. 

Ohms  per  Ih. 
approximate. 
Copper. 

Silk 
covered. 

Cotton 
covered. 

IO 

•134 

3'4°4 

6 

6 

•01058 

12 

•ICQ 

2770 

9 

9 

•02416 

14 

•083 

2-108 

15 

15 

•07186 

16 

•065 

I  '65  1 

25 

24 

•igiO 

18 

•049 

1-245 

45 

41 

•5916 

20 

•035 

•8890 

64 

59 

2-273 

22 

•028 

•7112 

119 

109 

5*548 

24 

X>22 

'   -5588 

167 

156 

I4"?6 

26 

•018 

•4571 

266 

244 

32'49 

28 

•014 

•3556 

414 

383 

88-77 

30 

'012 

•3048 

542 

500 

it>4'  5 

32 

•009 

•2286 

870 

800 

521-19 

34 

•007 

•1778 

1,170 

1,  080 

1421-98 

35 

•005 

'13 

1,475 

1,360 

5473-332 

36 

•004 

•1016 

i  ,660 

1,530 

13341.014 

In  calculating  from  this  table,  it  must  be  remembered 
that  the  resistance  of  any  wire  coiled  on  a  Siemen's  n: 
girder  form  of  armature,  is  proportional  to  the  length  of 
wire  used:  but  in  the  ring  armature  form  where  the 
winding  is  continuous,  the  resistance  is  only  £  that  of 
the  entire  length  of  wire  employed.  There  is  still  one 
consideration  to  be  made  in  deciding  upon  the  gauge  of 
wire  to  be  used  in  the  armature,  and  that  is,  that  the  cur- 
rent carried  must  in  no  case  be  so  great  as  to  heat  the 


7 HE  DYNAMO.  113 

wire.  It  is  usual  to  allow  I  square  inch  section  for  each 
2,000  amperes  to  be  carried  ;  and  this  rule  holds  good 
in  the  smaller  diameters. 

Based  on  these  rules,  the  armature  may  be  constructed 
as  small  as  will  hold  the  requisite  amount  of  wire.  The 
dynamo  figured  at  43,  is  capable  of  working  up  to  120 
c.p.,  say  six  lamps  of  50  ohm's,  cold  resistance,  requiring 
one  ampere  each  ==  6  amperes.  The  E.M.F.  of  this 
machine  should  be  about  50  volts.  In  Fig.  44,  owing  to 
the  fact  that  the  armature  is  made  deeper,  as  many  as 
ten  such  lamps  may  be  put  on.  In  winding  as  a  shunt 
machine,  it  will  be  well  to  put  the  following  rules  into 
practice.  1st.  Let  the  resistance  of  the  armature  coils 
(£  the  entire  length  of  wire)  be  -fa  of  the  resistance  of  the 
outer  circuit,  including,  lamps,  leads,  baths,  accumulators, 
etc.  2nd.  Let  the  resistance  of  the  field  magnet  coils  be 
2O  times  that  of  the  outer  circuit  3rd.  Let  the  diameter  of 
the  wire  chosen  to  fulfil  this  latter  condition  be  such,  that 
the  diameter  of  the  wound  fisld  magnets  does  not  exceed^ 
nor  fall  greatly  short  of,  twice  the  diameter  of  the  bare 
iron  cores. 

One  example,  worked  out  in  full,  is  here  give*  to 
show  how  these  data  are  to  be  employed  in  practise. 
Let  it  be  desired  to  construct  a  gramme  dynamo  to 
light  five  twenty  c.p.  lamps  of  50  ohms  cold  resistance, 
each  requiring  about  one  ampere  to  light  it  to  the 
full.  Supposing  the  resistance  of  the  "  leads  "  (cables, 
connections  etc.)  to  be  one  ohm.  The  highest  resis- 
tance we  shall  have  lo  overcome  in  the  outer  circuit  will 
be.  when  only  one  lamp  is  used,  and  this  will  not  exceed 


114  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 

50  ohms  for  the  lamp,  and  one  ohm  for  the  leads 
=  51  ohms.  In  this  case  one  ampere  only  will  be 
needed.  When  all  the  lamps  are  on,  in  parallel  circuit, 
the  resistance  falls  to  £  of  50  for  the  lamps  =  10  ohms, 
and  one  ohm  for  the  leads  =  1 1  ohms.  In  this  latter 
case,  however,  we  shall  need  five  amperes  to  feed  the 
five  lamps.  Hence  in  the  former  case,  if  the  armature 
be  wound  so  as  to  give  5 1  volts,  we  shall  get 

E  =  5 1  volts  < 

v, ,-        =  I  ampere  : 

R  =  5 1  ohms 

and  in  the  latter,  if  the  armature   be  wound  to  give 

55  volts,  we  shall  have 

E  =  55  volts 

_ — — r-      =5  amperes. 

R  =  1 1  ohms 

Hence,"  as  far  as  the  outer  circuit  is  concerned,  an 
E.M.F.  of  55  volts  will  be  ample.  It  will  be  well  to  allow 
a  trifle  more  than  this,  because  of  the  resistance  of  the 
armature.  Therefore,  allowing  one  yard  per  volt,  56  or  58 
yards  of  No.  18  wire  will  be  sufficient  on  the  armature. 
As  the  current  will  never  exceed  five  amperes,  the 
diameter  of  the  wire  need  not  exceed  No.  18  gauge,  as 
this*will  carry  safely  five  amperes  of  current.  Hence 

56  to  58  yards  of  No.  18  wire  will  suit  our  purpose  very 
well.     On   measurement  it  will  be  found  that  a  ring  of 
about  5  in.  diameter  will  take  this  amount  of  wire,  the 
resistance  of  which  (£  of  56  yards  =  14  yards  =  0*25  of 
an  ohm)  falls  considerably  within  the  limit  of  $,  of  the 
outer  circuit  resistance,     An  armature  of  this  size  will 
admit  of  field  magnets  having  cores  presenting  12  inches 
length  by  2  inches  diameter,  on  which  the  magnetising 


THE  DYNAMO.  115 

coils  may  be  \vound.  Since  the  armature  resistance  is 
only  0*25  cf  an  ohm,  we  muct  put  on  a  v/ire  having  400 
timecthic,  or  400  X  '25  =  100  ohms  ;  and  yet  shall  not, 
when  wound  on,  greatly  exceed  the  original  core  diameter. 
It  will  be  found  that  13  t'  14  Ibs.  of  No.  24  gauge  v/ill 
fulfil  the  requirements.  In  the  above  example,  the  very 
best  soft  iron  is  supposed  to  be  employed.  Any  devia- 
tion from  this  will  entail  the  employment  of  more  wire, 
both  in  the  armature  and  in  the  field  magnets. 

§  74.  As  to  the  mechanical  part  of  winding  the  cores 
and  armature,  the  following  points  must  be  observed 
to  obtain  the  best  results.  1st.  The  cores  of  the  F.M. 
must  be  carefully  taped  and  varnished  to  insure  insula- 
tion. 2nd.  The  armature  must* also  be  most  caref-ully 
taped  and  varnished.  No  part  of  the  iron,  where  the 
wire  has  to  bo  wound,  should  be  left  uncovered.  3rd 
Plaving  decided  into  how  many  sections  the  armature  is 
going  to  be  wound,  the  wire  needed  to  go  round  one 
section  must  be  measured  off,  and  the  whole  series  of 
sections  cut  off  to  the  same  length.  4th.  The  wire,  both 
on  the  F.M's.  and  on  the  armature,  must  be  wound  as 
tightly  and  smoothly  as  is  possible.  5th.  After  winding, 
the  armature  coils  should  be  soaked  in  a  thin  shellac 
varnish  (French  polish  does  admirably),  and  then  baked 
in  "a.  slow  oven  at  a  heat  not  exceeding  212°,  until  the 
varnish  is  quite  hard  and  dry. 

In  both  the  Gramme  and  the  Mather-Hopkinson 
type  of  dynamo,  the  field  magnet  bars  must  be  wound 
so  as  to  produce  consecutive  poles  at  the  top,  and 
consecutive  poles  at  the  bottom  of  the  machine. 


Ilfi  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 

The  commutator  must  have  as  many  divisions  as  there 
are  sections  in  the  armature. 

The  brushes  should  be  self-regulating  by  means  of 
spring  and  swivel,  for  pressure  ;  and  should  be  capable 
of  adjustment  by  means  of  a  set  screw,  for  the  best 
angle  of  commutation 

In  the  shunt  wound  machine,  the  two_ends  of  the 
field  magnet  coils  are  each  connected  to  one  of  the 
brushes,  and  these  latter  again  connected  to  the  two 
binding  screws,  which  form  the  terminals  of  the 
dynamo. 

On  no  account  should  the  armature  be  mounted  on  a 
wooden  hub.  This  was  a  favourite  mode  with  the  older 
makers,  but  it  is  to  be  condemned,  in  view  of  its 
mechanical  inefficiency,  and  more  so,  because  of  the 
resistance  it  opposes  to  the  escape  of  the  heat  generated 
by  the  Foucault's  currents.  Brass  or  gun  metal  spiders 
only  are  admissible.  Iron  or  any  other  magnetic  metal 
must  not  be  used,  either  for  the  spiders  or  for  the  bearings 
of  the  shaft,  as  they  short-circuit  the  magnetic  field,  to 
the  great  detriment  of  the  inductive  effect  of  the  field 
magnets  or  the  armature  coils.  The  collector  bars  of 
the  commutator  should  be  as  massive  as  possible,  and 
separated  from  each  other  by  air  spaces,  as  in  the 
Hochhausen  form. 

§  75.  AMMETERS.— These  instruments  are  intended 
to  measure  the  amount  of  current  in  amperes  passing 
through  any  given  circuit.  One  ampere  of  current, 
parsing  through  a  solution  of  sulphate  of  copper,  is 
capable  of  depositing  18-35  grains  of  copper  per  hour. 


AMMETERS.  117 

To  make  such  an  instrument,  suitable  for  the  general 
purposes  of  the  amateur,  and  capable  of  reading  from 
I  up  to  5  or  6  amperes,  the  following  mode  of  procedure 
may  be  adopted : 

§  76.  Procure  a  square  piece  of  deal  4  in.  x  4  in. 
x  j  in.,  smoothly  planed  up  ;  also  a  glass-capped  circular 
cardboard  box,  3  in.  diameter  by  ii  in.  deep,  inside 
measure.  (Such  boxes  are  sold  by  mineralogists,  etc., 
for  preserving  specimens.)  Push^the  cardboard  bottom 
out  of  the  box.  This  bottom  may  be  used  as  the 
"  dial ','  \vhereori  to  inscribe  the  degrees  of  current. 
Stain  the  wood  black,  or  any  desired  colour,  and  varnish 
jt.  Now  trim  the  edges  of  the  bottom  of  the  box  into 
an  exact  circle,  capable  of  entering  easily  into  the  box 
from  which  it  was  removed.  Describe  n°ar  its  circum- 
ference three  circles  in  ink,  with  a  pair  of  compasses. 
Let  these  circles  stand  about  £  in.  from  one  another. 
Describe  also  a  small  circle  about  a  \  in.  diameter  in  the 
centre  of  the  card,  to  serve  as  a  guide  for  a  central  hole, 
which  must  be  punched  out  with  a  \  in.  circular  punch. 

§  77.  Now  make  a  small  glass  pivot,  as  described  at 
§§  5  and"  6,  about  £  in.  long,  and  not  more  than  \  in 
diameter  at  its  base.  When  made,  test  it  for  freedom 
and  absence  of  friction  by  poising  it  on  the  point  of  a 
very  fine  needle,  the  head  of  which  is  stuck  in  a  bung. 
If  it  spins  round  quite  freely  without  any  hitch,  well  and 
good ;  if  not,  make  others  until  you  get  one  perfectly 
freely  suspended.  This  is  most  essential. 

Procure  a  strip  of  copper  sheet,  about  ,V  in.  thick, 
4  in.  long,  and  \  in.  wide.  Punch  three  \  in.  holes  in  it. 


1 18  ELECTRICAL  INSTRUMENT  MAKING  FOX  AAfA  TEUKS. 

one  at  each  extremity,  wherein  to  pass  the  binding 
screws,  and  one  in  the  centre.  Lay  this  strip  on  the 
little  wooden  square,  across  its  centre,  and  screw  it  down 
to  the  square  by  means  of  two  small  binding  screws, 
taking  care  that  there  is  space  left  for  the  cardboard 
box  to  stand  between  them  without  touching  either. 
Now  stick  a  fine,  bright,  and  sharp  No.  8  needle,  point 
upwards,  into  the  centre  of  the  square,  just  where  you 
have  punched  the  central  hole  in  the  copper  slip.  Care 
must  be  taken  that  this  be  perfectly  perpendicular,  and 
clears  the  glass  top  of  the  capped  box  by  about  £  in. 
when  this  latter  is  placed  over  it. 


45- 

$  78.  With  a  pair  of  round-nosed  pliers,  make,  from 
a  short  piece  of  No.  16  pianoforte  wire  (steel),  a  single 
coil  to  fit  tightly  round  the  base  of  the  glass  pivot, 
having  a  straight  \  in.  prolongation  on  either  side  of 
the  central  loop  or  coil,  as  shown  at  Fig.  45 ,  A.  This  must 
be  thoroughly  magnetised  by  rubbing  with  a  good  horse- 
shoe magnet,  and  then  fastened  with  a  drop  of  glue  to 
the  base  of  the  pivot,  extreme  care  being  taken  that  no 
gjue  touches  the  rim  of  the  pivot,  otherwise  it  will  not 
swing  freely.  The  pivot  must  now  be  placed  en  the 
needle  in  the  centre  of  the  square  ;  and  the  punched 


AMMETERS.  119 

circle,  which  is  to  serve  as  a  dial,  held  over  it,  and 
lowered  down  over  it  until  the  point  of  the  pivot  protrudes 
about  J  in.  above  the  central  aperture,  and  the  magne- 
tised needle  below  clears  the  card  also  by  about  i  in. 
The  dial  is  to  be  fixed  -in  this  position  by  gluing  three 
corks  of  the  right  height  to  the  square,  and  then  gluing 
the  card  on  to  them.  Here,  again,  care  must  be  taken 
that  the  pivot  does  not  touch  the  card  dial  at  any  point, 
and  also  that  the  magnetic  wire  needle  below  is  clear  of 
the  dial. 

A  small  pointer  in  black  paper,  straw,  or  any  other 
very  light  rigid  material,  about  ij  in.  long,  is  to  be 
lightly  glued  to  the  top  of  the  pivot  at  'right  angles  to 
the  needle  below.  The  glass-capped  box  may  now  be 
placed  over  all,  and  glued  to  the  square  by  running  a 
lit'tle  thin  glue  round  its  bottom  edge.  When  quite  dry, 
the  grading  maybe  proceeded  with. 

§  79.  To  grade  correctly,  at  least  10  quart  Daniell 
cells  will  be  needed.  These  must  be  freshly  made  up, 
and  filled  to  about  half  their  capacity.  A  decomposi- 
tion cell  (a  stoneware  basin  or  foot-bath  will  'do), 
capable  of  containing  two  squares  of  copper  sheet, 
facing  one  another,  at  least  6  in.  X  6  in.,  must 
next  be  provided,  and  filled  with  a  strong  solution  of 
copper  sulphate  slightly  acidulated  with  sulphuric 
acid. 

The  copper  plate  which  it  is  proposed  to  attach  to 
the  negative  pole  of  the  battery  (the  zinc  end),  having 
been  caref  illy  weighed  in  a  balance  capable  of  turning 
to  the  iic  of  a  grain,  and  its  exact  weight  noted,  a 


120  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 

Daniell  cell,  the  ammeter,  and  the  decomposition  cell 
are  arranged  in  series,  as  shown  at  B,  and  allowed  to 
work  for  one  hour  exactly.  At  the  end  of  that  time 
the  copper  plate  is  withdrawn,  washed  (not  rubbed),  and 
dried.  When  dry,  it  is  weighed,  and  the  increase  noted. 
If  exactly  18*35  grains,  well  and  good  ;  if  not,  the 
following  alterations  must  be  made  until  the  deposit 
reaches  exactly  this  amount. 

Should  the  deposit  be  less,  approach  the  plates 
in  the  decomposition  cells,  and  add  more  fluid  to  the 
battery  cell,  until  the  deposit  reaches  exactly  the 
18*35  grains  per  hour.  If  the  batteries  are  less  than 
quart  cells,  two,  coupled  for  quantity,  may  be  needed 
to  produce  the  desired  result.  Should  the  deposit 
be  greater,  remove  the  plates  in  the  decomposition 
cell  farther  apart,  in  either  case  carefully  noting 
the  distance.  Once  the  desired  result  attained,  the 
greatest  care  must  be  taken  to  maintain  all  in  statu 
quo,  and  the  exact  position  of  the  two  copper  plates  in 
the  decomposition  cell  being  marked,  the  temperature  of 
room  taken,  the  height  of  the  fluids  in  the  battery  cell, 
or  cells  noted,  etc.  One  pole  of  battery  is  now  detached 
from  the  ammeter,  and  this  latter  moved  round  until  the 
magnetic  needle  lies  quite  parallel  with  the  copper  strip, 
hence  the  pointer  exactly  at  right  angles  to  it  The 
spot  at  which  the  pointer  stands  is  marked  with  O.  The 
battery  is  again  attached  to  the  ammeter,  and  the 
deflection,  when  the  needle  comes  to  rest,  noted.  Thfs 
deflection  corresponds  to  one  ampere  of  current,  and 
Should  -be  .marked  as  such  on  the  dial.  To  get  the 


THE   VOLTMETER.  121 

corresponding  deflection  on  the  other  side,  the  poles  of 
the  battery  must  be  reversed.' 

All  the  other  degrees  may  be  got  in  precisely  similar 
manner,  multiples  of  i8'35  gr.  of  copper  being  considered 
and  counted  as  so  many  amperes,  more  cells  being 
connected  up,  in  parallel,  to  obtain  the  desired  weight 
of  deposit  per  hour. 

§  80.  THE  VOLTMETER. — This  instrument  serves,  as 
its  name  implies,  to  .measure  the  voltage,  or  electro- 
motive force ,  of  any  source  of  electricity.  Perhips  there 
is  no  subject  so  puzzling  to  the  electrical  student  as  the 
difference  between  electro-motive  force  (E.M.F.  as  it  is 
generally  abbreviated)  and  current.  As  it  is  essential 
to  have  clear  ideas  on  these  points,  in  order  to  grasp  the 
principles  on  which  the  construction  of  the  voltmeter  is 
based,  I  shall  deviate  somewhat  from  the  plan  hitherto 
followed  in  these  pages,  and  devote  a  few  lines  to  the 
consideration  of  what  E.M.F.  really  is. 

§  8 1.  According  to  the  present  state  of  our  know- 
ledge, the  phenomena  which  we  group  together  under 
the  name  of  "  electricity  "  are  simply  manifestations  of 
a  peculiar  mode  of  motion  in  the  ultimate  particles  ol 
bodies,  called  atoms.  What  the  nature  of  this  motion 
may  be,  whether  rotary,  undulatory,  vibratory,  etc.,  we 
are  as  yet  utterly  unable  to  say.  We  have,  however, 
at  our  disposal,  many  means  of  setting  up  this  motion, 
such  as,  for  in  stance,  friction  (and  many  other  mechani- 
cal movements),  cJiemical  action,  the  application  or  ab- 
straction of  heat  (itself  a  mode  of  motion),  or  of  .light 
(another  form  of  motion),  etc.  I  purp  sely  leave  out 


122  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 

magnetism  as  a  source  of  electricity,  since  magnetism  is 
itself  but  a  manifestation  of  electricity. 

Whichever  of  these  means  be  employed '  as  sources  of 
electricity,  we  find  (other  things  remaining  Jlie  same) 
that  an  increase  in  the  means  employed  is  followed  by 
an  increase  in  effect.  Now  the  means  employed,  whether 
they  be  friction,  motion  in  the  field  of  a  magnet,  chemi- 
cal -action,  or  heat,  are  called,  when  viewed  under  this 
aspect,  "  electro-motive  force,"  or  that  which  sets  up  an 
electrical  condition.  In  the  same  way  as  we  can  only 
measure  the  power  of  a  man/  or  of  a  steam-engine,  by 
the  work  performed  by  them  when  they  exert  their 
force,  so  we  can  only  measure  the  E.M.F.  of  any  given 
battery,  dynamo,  frictional  or  induction  machine  by  the 
electrical  work  it  can  perform.  It  is  optional  whether 
we  use  as  measures  the  chemical,  the  magnetic,  or  even 
the  mechanical  work  which  the  electricity  set  up  by  the 
given  means  can  perform.  As  a  matter  of  convenience, 
the  magnetic  work  performed  is  generally  preferred  as 
?»  means  of  measurement. 

$  «2.  From  a  study  of  Ohm's  law,  we  learn  that  the 
amount  of  current  set  up  in  any  given  circuit  is  equal 
to  the  "  electro-motive  force  divided  by  the  resistance  in 
that  circuit."  This  simply  means  that,  with  any  given 
force  setting  up  electricity,  the  current  will  be  greater 
as  the  resistance  is  less.  This  i  is  usually  expressed 

p  =  C.    '  From  this  it  is  equally  evident  that  if  we  know 

the  value~of  R  (the  resistance),  and  of  C  (the  current), 
iwe  can  always  calculate  the  value  of  E,  by  multiplying 


THE  VOLTMETER.  123 

R  by  C.  Moreover,  it  will  be  equally  evident  that,  if 
R  be  constant,  E  will  always  be  directly  proportional 
to  C.  To  prove  this,  let  us  give  an  actual  value  in 
figures  to  the  E.M.F.,  the  R  and  the  C.  Let  us  sup- 
pose, for  example,  that  we  are  working  with  a  battery 
having  a  known  E.M.F.  of  two  volts,  and  we  get  a  cur- 
rent of  one  ampere  when  working  through  a  total  resis- 
tance of  2  ohms.  This  would  actually  be  the  case, 

Now,  let  us  imagine  that 

using  another  source  of  electricity,  pitted  against  a  pre- 
cisely similar  total  resistance,  we  found  we  obtained  a 
current  of  10  amperes.  What  would  E.M.F.  be  ? 

Simply--  (C  =  10)  x  (R  =  2)  =  (E.M.F.  —  20). 
Here  we  see  that  as  the  current  Is  ten  times  as  large  as 
in  the  first  instance,  so  also  is  the  electro -motive  force. 
But  it  will  be  perfectly  evident,  that  as  the  current  would 
be  altered  in  amount  by  any  alteration  in  the  resistance, 
it  is  essential  that  any  resistance  in  circuit  should  be 
"kept  as  nearly  as  possible  invariable,  if  we  desire  to  have 
results  that  are  ev/en  approximately  correct. 

§  83.  Unfortunately,  no  two  batteries  or  sources  of 
electricity  have  precisely  the  same  resistance  ;  and  even 
if  they  had  at  one  instant,  this  would  be  found  to  vary 
during  work.  On  the  other  hand,  the  internal  resistance 
of  most  batteries  does  not  often  fall  below  0*0016  of  an 
ohm, 'nor  rise  above  2  ohms  per  cell  It  follows  from 
this,  that  if  we  make  the  resistance  of  the  voltmetei; 
very  large  in  proportion  to  the  possible  resistance  of  the 
battery,  we  can  reduce  the  possible  error  to  within  any 


«4  ELECTRICAL  1NSTR  UMENT  MAKING  FOR  A  MA  TEURS. 

desired  limits.  In  practice,  it  is  usual  to  allow  a  resis- 
tance of  50  ohms  for  every  volt  to  be  measured.  This 
gives  sufficient  accuracy  for  most  purposes,  as  the  error 
is  theo-^vithin  3  per  cent,  of  the  total  readings. 

§  84.  From  the  above,  it  follows  that  a  galvanometer 
having  a  resistance  large  in  proportion  to  the  current 
to  be  measured,  if  graded  to  indicate  the  current  deflec- 
tion which  corresponds  to  a  given  voltage,  can  be  used 
as  a  voltmeter.  Certain  precautions  are  necessary  in 
order  to  obtain  deflections  at  all  parts  of  the  scale,  suffi- 
ciently distant  from  one  another  to  be  easily  read.  The 
following  instructions  will  enable  the  amateur  to  con- 
struct an  instrument,  which,  if  used  intelligently,  and 
far  from  masses  of  iron  or  other  magnetic  bodies,  will 
give  results  which  for  the  measurement  of  low  E.M.F., 
say  from  I  to  5  volts,  far  exceed  in  accuracy  those  given 
by  m,ore  pretentious  apparatus.  As  there  are  several 
points  of  resemblance  between  the  ammeter  and  the 
voltmeter,  the  student  will  do  well  to  refer  to  §  75,  pre- 
vious to  making  the  voltmeter. 

§  85.  The  first  thing  needed  is  a  light  cardboard  01 
chip  box,  2j  in.  long,  by  \\  in.  wide,  and  £  in.  in 
height.  There  must  be  no  ends  to  this,  which  exactly 
resembles  in  size  and  shape  the 'sliding  cover  of  a 
Swedish  "  Tandstick  "  box.  Indeed,  such  a  cover  may 
be  used  for  the  purpose  with  good  results.  If  the 
amateur  desires  to  make  this  himself  he  need  only  take 
a  strip  of  stoutish  millboard,  4$  in  Jong,  by  2\  in.  wide, 
fold  it  three  times  on  itself,  and  gJue  the  top  and 
side  edge  as  shown  in  figure  46.^  A  circular  central 


THE  VOLTMETER. 


12$ 


aperture,  -about  J  in.  in  diameter,  is  to  be  cut  in  the 
bottom  of  the  box  with  a  sharp  penknife,  while  a  central 
strip,  also  about  \  in.  wide,  but  extending  the  whole 
width  of  the  box,  is  cut  away  from  its  upper  portion. 
This  being  done,  the  box  is  slightly  moistened  with  thin 
glue,  and  then  quickly,  but  carefully,  wound  with  about 
j  oz.  of  No.  40  silk-covered  German  silver  wire.  This 
should  be  about  '0058  of  an  inch  in  diameter,  and  have 


Fio.  46. 

a  resistance  of  not  less  than  2570  ohms  to  the  ounce. 
Care  should  be  taken  to  wind  this  wire  evenly,  closely, 
and  without  kinks,  leaving  about  3  in.  wire  free  at  each 
end.  Having  started  winding  at  one  end,  the  operator 
proceeds  to  wind  continuously  across  the  box,  coil  after 
coil,  till  he  reaches  the  central  hole  and  slit ;  these  he 
leaves  open,  crossing  over  to  the  other  half  by  letting  the 
wire  pass  diagonally  across  one  side.  He  continues  the 
winding  on  to  the  othehside,  in  the  same  direction,  tiU 


126  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMAT&VRfy 

he  reaches  the  end  of  the  box.  If  the  glue  gets  dry 
during  the  winding  he  may  apply  a  little  more  fresh  glue 
as  he  nears  the  end,  so  as  to  ensure  that  the  wire  should 
not  uncoil  when  released.  When  the  box  has  been 
wound  from  end  to  end,  as  shown  fit  Fig.  47,  it  must  be 
allowed  to  dry  thoroughly  in  a  warm  dry  place,  and 
when  quite  dry  soaked  for  a  few  minutes  in  hot  melted 
paraffin,  until  permeated  with  it,  and  then  hung  up  to 


Fio.  47. 

'drain  and  set.  While  this  is  taking  place,  a  small  base 
;board  of  \  in.  deal  or  mahogany  is  planed  up  and  stained 
or  polished  as  desired.  This  piece  should  be  4  in. 
square.  The  exact  centre  of  this  having  been  found,  a 
needle  l-r  in.  in  length  is  forced  head  downwards  in  the 
iboarJ  at  this  spot  so  that  the  pointed  end  stands 
perpendicularly  upwards  at  a  height  of  i£  in.  from  the 
surface  of  the  base  board.  A  small  I  in.  magnetic 
needle  is  now  made  out  of  a  short  length  of  piano  wire, 


THE  VOLTMETER.  127 

and  fastened  to  a  glass  pivot  precisely  as  directed  in  the 
case  of  the  ammeter  (see  §  77  and  78).  This  is  carefully 
tried  for- freedom  of  motion,  equality  of  magnetism,  etc. 
The  box,  coiled  with  wire,  is  now  placed  over  the  upright 
needles  on  the  board  (this  latter  being  made  to  pass 
through  the  central  apertures,  without  touching  them, 
so  as  not  to  blunt  or  grease  the  needle  point,  which 
would  be  fatal  to  free  movement),  and  while  in  this 
position,  the  pivoted  magnetised  needle  placed  on  the 
point,  so  as  to  see  whether  the  magnet  swings  freely 
without  touching  any  part  of  the  inside  of  the  box,  of 
which,  in  fact,  it  ought  to  stand  in  the  centre.  It  may 
require  raising  a  trifle,  which  may  be  done  by  means  of 
thin  slices  of  cork.  When  this  has  been  duly  adjusted, 
the  cork  slices  (if  any)  and  the  box  are  fastened  down  to 
the  base  board  with  a  little  good,  hot  glue.  Care  must 
be  taken  that  the  coiled  box  stands  squarely  on  the 
base,  with  its  longest  edge  parallel  with  one  side  of 
the  base  board,  and  with  the  poising  needle  passing  up 
the  middle  of  the'  central  circular  hole.  When  this  has 
been  arranged,  the  glue  is  allowed  to  set  firmly.  The 
next  operation  is  to  bore  two  small  holes  right  through 
the  base  board — one  at  each  extremity  of  the  coiled 
box — to  allow  the  projecting  ends  of  wire  to  pass  through. 
These  holes,  as  v/ell  as  one  at  each  of  two  corners  of  the 
base  board  in  which  are  to  be  inserted  the  terminal 
binding  screws,  are  best  made  with  a  small  Archimedean 
drill.  With  a  sharp  penknife,  two  little  channels  are  cut 
in  the  under  surface  of  the  base  board  leading  from  the 
first  two  holes  to  the  latter  two.  The  ends  of  the  wire 


128  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS*. 

on  the  coiled  box  are  then  passed  through  the  more 
central  hole,  and  the  extremities,  having  been  stripped 
onXiheir  silken  covering,  pushed  through  the  corner  holes 
so  as  to  return  to  the  upper  surface  of  the  board.  They 
are  held  in  this  position  v/hile  a  binding  screw  is  screwed 
in  each  of  the  two  corner  holes.  This  having  been 
effected,  any  excess  of  wire  above  is  cut  off,  and  the 
portion  below  caused  to,  make  perfect  contact  with  the 
tang  of  the  binding  screw  by  a  touch  with  a  drop  of* 
solder  on^ a  <hot  iron.i  The  .channel  in  which  the  wire 
ends  lie,junder^the.base  board,  ^should  jiow-  beJUledJaj 


PIG.  48. 

with  a-little  hot  guttapercha,  or  Prout's  glue  (see  Fig.  48)} 
The  pivoted  needle  may  now  be  definitely  placed  on  thq 
point.  A  circle  of  about  3  in.  in  diameter  is  now  cut 
out  of  a  clean  stout  white  card,  and  two  inner  circles 
described  with  the  compasses  and  good  black  inlc, 
within.  One  should  be  \  in.  less  all  round  than  the 
card,  and  the  other  \  in.  all  round  less  than  the 
former.  For  convenience  of  grading,  a  central  line 
should  be  drawn  across  the  card,  so  as  to  divide  it  into! 
two  equal  semicircles.  One  end  of  this  line  should  bej 
marked  o. 


THE  VOLTMETER. 


129 


A  perfectly  round  central  \  in.  hole  should  now  be 
punched  in  this  card,  which  must  then  be  glued  on  to 
the  coiled  box,  great  care  being  taken  to  have  the  central 
O  line  parallel  with  the  wire  coiled  round  the  box,  and 
therefore  parallel  with  the  centre  slit.  Care  also  must 
be  taken  that  no  glue,  etc.,  be  allowed  to  touch  the  glass 
pivot,  the  apex  of  which  must  protrude  centrally  through 
the  hole  in  the  card,  and  stand  above  about  \  in.  To 
insure  the  card  drying  flat,  it  is  well  to  place  two  or 
three  half  bullets,  or  similar  weights,  on  it  until  dry. 

A  small  pointer,  made  either  of  stout  glazed  black 
paper  or  of  blackened  straw,  about  ij  in.  in  length,  is 
now  glued  to  the  top  of  the  pivot,  care  being  taken  that 
it  is  exactly  parallel  with  the  magnetised  needle  below. 
It  is  not  difficult  to  secure  this  parallelism  if  the 
magnetised  needle  be  held  perfectly  at  right  angles  with 
the  zero  line  by  means  of  a  bar  magnet  inserted  inside 
the  coiled  box,  while  the  pointer  is  being  glued  on 
outside,  also  at  right  angles  to  this  zero  line. 

The  pointer  must  be  allowed  to  dry  thoroughly  before 
any  attempt  at  grading,  etc.,  be  made.  When  quite 
firm  in  its  position,  a  3  in.  glass-capped  box  (§  76) 
about  l£  in.  deep,  must  be  selected,  and  glued  on  to 
the  base  board,  the  bottom  of  the  box  having  previously 
been  removed,  as  described  in  the  case  of  the  ammeter. 
This  box,  with  its  movable  glass-capped  lid,  serves  to 
keep  the  instrument  free  from  dust,  and  unaffected  by 
currents  of  air. 

To  grade  this  instrument,  it  is  necessary  to  have  access 
to  five  freshly- charged  clean  Daniell  cells.  (See  §  79;) 
K 


130  EtECTRICAL  INSfR UMENT MAKING  FOR  AMA  TE  U$S. 

These  give  a  current  of  about  I '079  volt  per  cell,  and 
before  using  them  definitely  they  should  be  separately 
coupled  up  to  the  voltmeter,  to  see  if  the  deflection 
given  by  each  cell  is  sensibly  the  same.  Supposing 
this  to  be  the  case,  the  voltmeter  is  allowed  to  come 
to  rest,  turning  the  instrument  until  the  pointer  stands 
exactly  at  zero.  (The  instrument  should  be  held  firmly 
in  this  position  during  the  following  trials.)  When  this 
takes  place  one  cell  of  the  Daniells  is  to  be  coupled 
up  to  the  terminals  of  the  voltmeter,  and  the  deflection 
noted.  With  a  sharp  pointed  HB  pencil  (the  glass 
cover  having  been  previously  removed),  a  dot  is  made 
on  the  end  at  the  spot  where  the  pointer  indicates. 
This  is  one  volt,  and  very  nearly  i-i2th  over;  more 
exactly,  ixftfo,  if  the  battery  be  in  perfect  condition  ; 
but  it  may  fall  as  low  as  irgfo  when  the  copper  solution 
works  through  the  porous  cell.  Having  made  this  mark, 
the  battery  is  uncoupled  from  the  terminals,  and  the 
C9nnections  reversed,  so  as  to  get  a  deflection  in  the 
opposite  direction.  When  the  needle  has  come  to  rest 
(with  the  glass  cover  over),  the  deflection  is  noted  as 
before,  and,  as  before,  a  dot  made  on  the  card  where 
the  pointer  stands.  Two  cells  are  now  coupled  in 
scrie(s,  and  the  same  operations  gone  through  to  get 
a  second  set  of  dots,  which  stand  for  2  volts  and,  say,  £. 
In  like  manner,  one  -after  the  other,  the  whole  5  cells 
are  coupled  in  series,  and  connected  to  the  voltmeter, 
and  the  relative  deflections  noted.  Lines  can  then  be 
ruled  from  the  circumference  towards  the  centre  of  the 
card,  In  ink,  allowing  for  the  one-  twelfth  excess  on  each 


GALVANOMETERS.  I3I 

dot,  and  these  lines  may  be  numbered  I,  2,  3,  4,  5,  and 
they  will  indicate,  with  very  fair  accuracy,  the  deflection 
given  by,  a  corresponding  number  of  volts.  At  Fig1.  49  is 
shown  the  mode  in  which  the  card  dial  is  divided, 
centred,  and  finally  graded.  Each  instrument  will  vary 
a  little  in  its  degrees  owing  to  the  different  resistance 
of  the  wires,  their  -distance  from  the  magnetic  needle,  etc.; 


Fia.  49. 

but  once  graded,  they  will  be  found  to  remain  very 
fairly  constant  in  their  indications  as  long  as  the  pivot 
remains  uninjured. 

§  86.  GALVANOMETERS. — Besides  the  ammeter  and 
voltmeter  described  in  our  last  sections,  there  are  two 
other  forms  of  galvanometer  which,  although  giving  no 
absolute  measure  in  currentror  voltage  (unless  calibrated) 
are  extremely  useful  for  the  purposes  of  detecting 


132  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 

weak  currents,  or  comparting  their  forces.  These  are 
the  ordinary  single  and  double  needle  galvanometer 
(also  called  multiplier  and  galvanoscope),  and  the 
tangent  galvanometer. 

§  87.  A  very  convenient  form,  of  single  needle  gal- 
vanometer may  be  constructed  on  precisely  the  same 
lines  as  given  at  $  85  for  the  voltmeter ;  the  only 
difference  being,  that  >silk  covered  copper  wire  should 
be  substituted  *for  the  German  silver  wire,  therein 
suggested.  The  diameter  of  the  wire,  and  the  quantity 
to  be  employed;  will  depend  entirely  on  the  use  to 
which  it  is  intended  to  put  the  galvanometer  when 
completed.  I/  it  is  to  be  used  for  large  currents  of 
very  low  tension  (such  as  thermo  currents,  etc.),  it  will 
be  better  to  use  a  few  turns  of  No.  16  wire.  If  on  the 
contrary  small  currents  of  higher  tension  are  to  be 
detected,  then  wire  as  fine  as  No.  40,  may  be  coiled 
round  the  frame  br  box.  A  very  useful  all  round 
instrument  may  be  constructed,  by  winding  about  200 
turns  of  No.  36  silk  covered  copper  wire  round  the 
frame,  or  box.  Th,is  will  produce  a  galvanometer  that 
will  give  a  deflection  of  about  20°,  with  the  current  set 
up  by  heating  the  junction  of  a  6"  length  of  copper 
wire,  twisted  to  a  similar  piece  of  iron  wire.  Of  course, 
instead  of  using  &  card  dial  as  shown  at  §  85,  graded 
to  volts,  the  divisions  of  the  circle  should  be  in  360°, 
numbered  from  o°  to  90°  on  either  side  of  the  zero 
line. 

$  88.  If,  instead  of  poising  the  needle  on  a  point  and 
pivot,  it  be  suspended  by  a  single  fibre  of  silk,  depend- 


GALVANOMETERS.  133 

ing  from  a  brass  rod,  bent  at  right  angles  like  an  P,  the 
short  arm  standing  about  6  in.  above  the  centre  of  the 
coiled  box,  a  very  much  mere  cencitive  instrument  is  the 
result.  In  this  case  the  magnetised  neddle  had  better 
be  a  well  magnetised  sewing-needle,  broken  off  at  its 
point  to  the  length  required.  This  should  be  thrust 
half  through  a  fine  straw,  about  I  in.  long,  at  right 
angles  to  the  length  of  the  straw,  and  nearly  at  one 
extremity  of  it,  similar  to  a  ±.  The  silken  fibre  can  be 
attached  to  the  opposite  end  of  the  straw  by  means  of  a 
drop  of  sealing-wax.  A  similar  needle  (not  magnetised) 
is  now  thrust  through  the  opposite  end  of  the  straw, 
perfectly  in  the  came  plane,  and  parallel  to  the  lower 
needle,  so  that  the  combination  presents  the  appearance 
of  a  letter  X .  This  arrangement,  hanging  from  the  bent 
brass  rod  over  the  central  clot  in  the  coilec^box,  must  be 
fixed  to  the  brass  rod  by  the  silken  fibre,  at  such  a  height 
that  the  magnetised  needle  enters  the  slot,  and  hangs 
equi-distant  from  the  top  and  bottom  cf  the  box.  The 
upper  needle,  or  "  pointer,"  should  clear  the  upper  sur- 
face of  the  coiled  box  by  nearly  a  half  inch.  Since  it 
would  be  impossible  to  fasten  the  graded  circle  on  the 
box  when  the  needles  are  in  position  if  the  circle  is 
entire,  we  "must  cut  the  circle  into  two  halves  down  the 
zero  line,  and  glue  the  two  halves  together  again  on  the 
box  by  the  under  side.  A  glass  shade  placed  over  the 
whole  will  protect  the  needles  from  air  currents. 

§  89.  If  in  either  of  the  two  last-described  instruments 
the  "  pointer ' '  be  replaced  by  a  carefully  magnetised 
needle,  of  the  same  size  and  "moment  "  as  the  under 


134  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 

one,  placed  with  its  north  pole  over  the  south  pole  of  the 
lower  needle,  and  vice  versd,  we  have  the  "astatic 
galvanometer."  Owing  to  the  (almost)  complete  neutral- 
isation'of  the  earth's  pull  on  the  magnets,  and  to  the 
increased  effect  of  both  upper  and  under  surface  of  coiled 
box  on  the  two  magnets,  the  sensitiveness  of  this  in- 
strument is  much  greater  than  that  of  any  of  the 
preceding. 

$  90.  The  tangent  galvanometer  presents  no  difficulty 
in  construction.  A  small  lozenge-shaped  "needle "is 
made  from  a  thin  piece  of  watch  spring,  about  I  in.  long 
and  j  in.  wide.  This  is  "  let  down,"  or  softened,  by 
being  held  over  the  flame  of  a  spirit  lamp  until  of  a  dull 
red,  and  allowed  to  cool  gradually.  When  quite  cold 
a  small  hole  A  in.  in  diameter  is  drilled  through  the 
centre.  The  *  needle"  is  then  straightened  out,  and 
tested  for  centrality;  and,  if  defective,  filed  until  the 
hole  corresponds  with  the  centre  of  gravity.  It  is  then 
hardened  by  being  made  nearly  red  hot  over  the  flame 
of  a  spirit  lamp,  and  being  dropped  into  cold  water.  It 
must  then  be  carefully  magnetised  by  being  rubbed  at 
each  extremity  with  the  opposite  poles  of  a  good  horse- 
shoe magnet.  When  fully  magnetised  it  must  be  fitted 
with  a  small  glass  pivot,  made  as  described  at  §  6,  small 
enough  to  enter  the  A  in.  hole  in  the  needle,  and  about 
J  in.  in  length.  Great  care  must  be  exercised  in  the 
choice  of  a  pivot,  which  must  be  very  perfectly  shaped, 
so  as  to  allow  great  freedom  of  motion  in  the  poised 
needle.  This  point  being  settled,  the  pivot  is  attached 
to  the  needle  by  means  of  a  mere  trace  of  good  glue, 


GALVANOMETERS.  135 

applied  to  the  hole  in  the  needle  only.  The  needle 
must  now  be  poised  by  its  pivot  on  a  fine  steel  sewing 
needle  (No.  IO  will  do),  and  any  want  of  perfect  hori- 
zontality  must  be  remedied  while  the  glue  is  still  moist. 
When  the  above  is  quite  dry,  a  very  fine  straw,  about 
i\  in.  long,  has  a  small  hole  made  in  its  centre  (half 
way  between  its  two  extremities)  with  a  rather  coarse 
pin  ;  then  the  head  of  the  pivot  is  pushed  through  this 
hole  in  the  straw,  so  as  to  cause  the  straw  to  lie  exactly 
at  right  angles  over  the  needle.  The  merest  trace  of 
glue  will  now  cause  the  straw  to  adhere  to  and  retain  its 
position  on  the  glass  pivot.  Thjs  can  now  be  set  aside 
to  dry.  While  this  is  drying  the  base  board  and 
standard  can  be  fitted  up.  A  piece  of  good  dry 
wood,  either  deal  or  mahogany,  is  planed  up  and  cut 
into  a  slab  6  in.  x  6  in.  x  £  in.  In  the  centre  of  this 
is  inserted  a  circular  wooden  rod,  I  in.  in  diameter,  and 
6  in.  in  length.  To  the  top  of  this  rod  is  glued  one  of 
the  glass-capped  boxes  mentioned  at  §76;*  which  in 
this  case  should  be  at  least  3  in.  in  diameter,  but  need 
not  exceed  £  in.  in  depth.  This  should  stand  squarely 
and  centrally  on  the  top  of  the  rod,  like  the  cap  ol  a 
mushroom  on  its  stalk.  When  the  glue  has  dried,  and 
the  box  is  firm  in  it?  place,  the  exact  centre  of  the 
bottom  of  the  box  is  found  by  a  pair  of  compasses,  a 
small  hole  being  made  with  a  sharp  needle  to  mark  the 
spot.  A  No.  10  needle  is  then  driven  (head  down- 
wards) through  the  centre  of  the  bottom  of  the  box  into 
the  wooden  rod  below,  until  the  point  of  the  nfeedle  is 
out  \  in.  below  the  glass  of  the  cover  when  the  cover 


136  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 

is  put  on  ;  more  exactly,  the  point  of  the  needle  must 
stand  at  such  a  height  that  when  the  pivoted  magnetic 
needle  and  straw  pointer  is  placed  on  it  the  whole 
system  shall  ride  clear  of  the  glass  cover  when  this  is 
placed  on  the  box.  Before  placing  the  magnetised  needle 
permanently  in  position,  a  white  paper  circle  of  the 
same  diameter  as  the  interior  of  the  box,  and  graded 
from  o°  to  900  on  either  side  of  the  zero  line,  should  be 
pushed  over  the  needle  point  and  glued  to  the  bottom 
of  the  box.  (The  greatest  care  must  be  taken  that  no 
glue  gets  on  the  needle  or  pivot,  as  all  freedom  of 
motion  would  thereby  be  destroyed.) 

§  91.  A  band  or  strip  of  copper,  about  I  in.  wide  by 
A  in.  thick,  and  3  ft.  4  in.  Jong,  is  bent  so  as  to  form  a 
circle,  I  ft.  in  diameter,  with  side  extensions,  like  the 
Greek  letter  Q.  Two  holes  must  be  put  through  the 
extended  extremities,  to  allow  01"  two  binding  screws 
being  inserted,  which  binding  screws  serve  at  one  and 
the  same  time  to  make  contact  with  the  copper  ring,  and 
to  hold  this  latter  to  its  place  on  the  base  board.  The 
position  of  the  ring,  with  regard  to  the  graded  dial  at 
its  centre,  should  be  such  that  the  90°  line  lies  parallel 
to  the  plane  of  the  ring;  so  that,  in  fact,  when  the 
magnetised  needle  is  parallel  with  the  ring,  the  straw 
pointer  s'tattds  at  zero.  This  constitutes  all  that  is 
necessary  to  the  efficient  action  of  the  tangent  galvano- 
meter. Of  course,  the  stand  may  be  polished,  or  stained 
and  varnished  ;  if  required  more  sensitive,  a  few  turns  of 
No.  16  copper  wire  (single  silk  covered),  maybe  substi- 
tuted lor  the  copper  ring.  If  currents  of  high  tension 


GAL  VANOME  TERS. 


137 


are  to  be  measured,  the  copper  band  must  be  replaced 
by  a  light  wooden  frame  (like  a  tambourine  frame),  on 
which  must  be  wound  several  hundred  coils  of  fine 
insulated  copper,  or  even  German  silver  wire.  When 
well  made,  the  tangent  galvanometer  indicates  the  force 
of  the  current,  as  the  "  tangent  of  the  angles  of  deflec- 
tion." As  this  may  not  be  quite  clear  to  the  non- 
mathematical  amateur,  I  append  a  table,  in  which  the 
relative  values  of  the  angular  deflections  are  approxi- 
mately expressed  in  degrees  of  force  : — 


Amount  of  Deflection  in  degrees 

of  Arc. 

Relative  forces  of  Currents  producing 
deflections. 

I 

rooo 

5 

5-012 

10 

10-118 

IS 

15-347 

20 

20-847 

25 

26-814 

30 

33-076 

35 

40-114 

40 

48-066 

45 

57-290 

50 

68-275 

u 

81-818 

99-220 

65 

122857 

70 

157-398 

75 

213-836 

80 

324-867 

I3 

654-824 

89 

3281-871 

9° 

Infinite. 

If  one  semi-diameter  of  the  graded  circle  be  divided 
into  degrees  of  arc  from  o°  to  90°  on  either  side  of  the 


1*38  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMA  TEURS. 

zero  line,  and  the  other  semi-diameter  divided  in  the 
same  manner,  but  numbered  with  the  integers  showing 
the  corresponding  amounts  of  force,  the  readings  may  be 
taken  from  this  latter  scale,  without  appreciable  error, 
and  without  necessitating  a  fresh  calculation  for  each 
experiment.  When  absolute  accuracy  is  required, 
reference  must  be  made  to  a  table  of  "Natural 
Tangents."  A  glass  shade  to  cover  the  whole  is  essen- 
tial, if  the  glass  capped  box  be  not  used. 

§  92.  The  previously  described  galvanometers  are 
frequently  required  in  testing  the  resistances  of  different 


lengths  of  line,  or  of  different  samples  of  wire,  lamps, 
etc.,  in  which  case  some  means  by  which  these  unknown 
resistances  may  be  compared  to  a  known  one  must  be 
adopted.  The  instrument  usually  employed  for  this 
purpose  is  the  "  Wheatstone  Bridge  "  or  balance. 

The  Wheatstone  Bridge  consists  in  a  dry  seasoned 
piece  of  wood,  nicely  planed,  stained,  and  varnished,  2  ft. 
long,  4  in.  wide,  by  j  in.  thick,  as  shown  at  Fig.  50.  Two 
strips  of  thin  copper,  B  B,  of  shape  of  the  letter  -j,  are 
fastened,  one  at  each  end  of  the  base  board.  These 
strips  are  cut  J  in.  wide,  and  are  si  in.  bng  in  their 


TTHE  THERMOPILE.  139 

longest  arm,  and  2  in.  in  their'  shortest.  Another 
straight  strip,  of  the  same  width,  but  1 6  in.  long,  is 
placed  between  these  two  short  arms,  leaving  2  in.  gap 
on  either  side.  These  strips  are  fastened  down  in  their 
places  by  nine  telephone  binding  screws,  as  shown  at  CCft 
E,  F  F',  G  G',  and  H  H'.  A  thin  German  silver  wire 
(No.  30)  connects  the  binder  C,  to  c',  and  a  small 
wooden  block,  carrying  a  spring  contact  piece,  runs 
along  the  v/ire.  A  tenth  binding  screw,  D,  serves  to 
connect  this  to  one  pole  of  the  battery,  E  being 
connected  to  the  other.  The  galvanometer  is  con-i 
nected  to  F  and  F*,  while  the  known  and  uflknown 
resistances  are  placed  betv/cen  G  G'  and  H  H'.  Just 
below  the  wire  which  is  stretched  between  the  two 
binding  screws  is  gummed  a  piece  of  white  paper  about 
^  in.  or  i  in.  wide.  In  the  exact  centre  is  marked  the. 
zero  line,  and  the  portions  to  the  right  and  left  of  this 
are  divided  into  10,  20,  100,  or  500  divisions,  according 
to  delicacy  required. 

§  93.  THE  THERMOPILE. — This  instrument  depends 
in  its  action  on  the  fact  that  when  the  point  of  junction 
between  two  pieces  of  metal  is  heated,  or  cooled,  a 
current  of  electricity  is  set  up.  It  is  not  necessary  to 
this  effect  that  the  metals  should  be  of  different  nature ; 
but  they  must  bs  in  different  molecular  states,  so  that 
they  oppose  different  resistance  to  the  passage  of  heat 
and  electricity.  As,  hov/ever,  the  greater  the  difference 
in  these  respects  the  greater  the  force  called  into  play, 
it  is  usual  to  employ  dissimilar  metals,  or  metallic  alloys^ 
etc.,  in  which  these  differences  are  highly  accentuated* 


!40  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 

$  94.  A  very  convenient  form  of  thermopile  for  the 
imateur,  and  one  which,  with  a  little  care  in  the  manage- 
ment of  the  heat,  gives  a  very  constant  current,  adapted  to 
delicate  and  long-continued  experiments,  such  as  grading 
delicate  galvanometers,  etc.,  may  be  constructed  as 
follows :  Cut  25  or  30  pieces  of  No.  16  German  silver 
wire  into  6  in.  lengths  ;  do  likewise  with  a  similar 
number  of  pieces  of  No.  16  copper  wire.  With  a  pair 
of  pliers  twist  tightly  about  I  in.  of  the  extremity  of  one 
copper  wire  to  the  extremity  of  one  German  silver  wire, 


FIG.  51. 

so  as  to  join  them  firmly  together,  as  shown  at  Fig.  51  A 
In  a  similar  manner  join  the  remaining  pairs  of  copper 
and  German  silver  wires  ;  clean  the  junctions  with  a  bit 
of  sandpaper,  and  solder  the  junctions  with  as  little  hard 
solder  as  you  possibly  can.  Now  twist  and  solder  the 
opposite  extremities  of  the  joined  wires,  each  German  silver 
to  the  copper  of  the  next  pair  (not  its  own),  leaving  the 
first  copper  and  last  German  silver  as  terminals  (see 
Fig.  51  B).  Now  procure  two  wooden  cornice-pole  rings, 
about  3£  in.  in  diameter  (such  as  are  used  to  suspend 
burtains  from  the  pole),  and  with  a  coarse  rasp,  file  one 


THE  THERMOPILE.  I4i 

surface  of  each  ringjfo/,so  that  if  laid  upon  one  another, 
after  filing  down,  they  form  one  thick  ring,  .is  sfcown  at 
Fig.  52.  To  the  lower  ring,  at  equi-distant  points, 
should  be  affixed  three  wooden  rods,  about  6  in.  long, 
spreading  out  somewhat,  so  as  to  admit  of  the  insertion 
of  a  spirit  lamp  between  them.  For  the  sake  of  stability, 
these  rods,  which  form  the  feet  on  which  the  thermopile 
rests,  may  be  inserted  into  a  heavy  wooden  or  leaden 
base  as  shown  at  D  in  Fig.  53.  The  next  step  is  to 
place  the  joined  wires  in  a  radiating  fashion  on  the  ring, 


Fid.  52.. 

and  clamp'them  in  their  place  by  covering  them  with 
the  upper,  ring,  which  must  be  screwed  tightly  down  to 
the  lower  one,  care  being  taken  that  the  screws  do  not 
tpuch  the  wires,  and  also  that  the  wires  do  not  touch 
each  other,  except  at  the  soldered  junction.  The  inner 
circle  of  junctions^must  not  touch^but  must  stand  at 
equal  distances  all  round  the  centre  of  the  circle  described 
oy  the  ring,  leaving  a  space  of  about  \  in.,  in  which  the 
flame  of  the  spirit  lamp  can  burn  and  heat  these 
junctions  simultaneously.  The  two  unsoldered  ends  (of 
fcopper  and  German,  silver  wires)  are  then  to  be  attached 


t4*  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 

to  binding  screws,  which  serve  as  the  poles  or  terminals 
of  the  thermopile.  Fig.  54  illustrates  the  appearance  of 
the  finished  instrument  On  plating  a  lighted  spirit 
lamp  on  the  base,  so  that  the  flame  plays  against  the 
junctions  pointing  to  the  centre,  a  current  will  be  found 
to  flow  from  the  two  terminals.  The  electro-motive  force 
of  these  little  instruments  is  about  one-twelfth  of  a  volt 
D 


FIG.  53. 

for  each  pair  of  wires,  so  that  a  dozen  pairs  of  wires 
give  very  nearly  one  volt.  On  the  short  circuit  each 
fair  is  capable  of  giving  about  one-third  of  an  ampere. 
Coupled  up  in  series  (as  shown),  this  remains,  of  course, 
the  same.  To  increase  the  current,  the  elements  must 
be  made  larger,  or,  what  amounts  to  the  same  thing, 
coupled  up  in  parallel  instead  of  in  series.  For  the 


TffE  THERMOPILE.  M3 

benefit  of  those  amateurs  v/ho  may  be  desirous  of 
attempting  something  on  a  larger  scale*,  the  following 
account  of  a  thermopile  which  was  made  on  the  plan  of 
Clamond's  pile,  with  some  slight  modifications,  is  sub- 
joined:— 288  strips  of  tinned  iron,  I  in.  wide  by  5  in.  long, 
were  cut  out.  A  mould  ofplaster-of-Paris  was  prepared 


Pro.  54. 

in  which  could  be  cast  oblong  squares,  2^  in  long  by  I  in. 
in  square  section.  At  end  of  the  two  extremities  of  the 
mould  were  placed,  standing  upright,  a  tinned  iron  strip. 
The  mould  was  then  filled  with  a  molten  alloy,  consisting 
of  two  parts  of  antimony,  melted  with  one  part  of  zinc 
(as  this  alloy  expands  considerably  on  cooling,  it  must 


144  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 

be  withdrawn  from  the  mould  as  soon  as  set).  B^  this 
means  were  obtained  144  castings,  having  a  tinned  iron 
lug  at  each  end,  looking  something  like  a  letter  E,  withr 
out  the  central  stroke. .  An  "iron  ring,  about  I  in.  wide, 
\  in.  thick,  and  I  ft  iri  internal  diameter,  was  supported  on 
four  stout  iron  rods,  screwed  to  it,  at  a  distance  of  about 
10  ins.  frorri  the  ground.  On  this  ring  was  placed  a 
coating  of  good  Portland  cement,  about  \  in.  thick.  The 
(castings,  with  their  lugs  previously  bent,  as  shown  at 


FIG.  55. 


Fig.  55,  and  having  a  thin  piece  of  mica  (talc)  inserted 
between  the  inner  strips  and  the  alloy,  to  prevent 
contact,  were  then  arranged  in  a  circle  on  the  iron  ring, 
each  one  being  separated  from  its  neighbour  by  a  thin 
piece  of  mica,  and  the  whole  held  together  by  means  of 
a  little  Portland  cement  put  in  between  as  each  casting 
was  placed  in  position.  Several  such  layers  of  circles  of 
castings  were  thus  arranged,  care  being  taken  that  the 
faces  of  the  castings  projected  about  \  in.  inward  beyond 
fine  iron  ring.  When  the  entire  series  of  castings  had 


THE  THERMOPILE.  145 

been  thus  arranged  in  circles,  and  set  in  cement,  a  final 
layer  of  cement,  about  \  in.  thick,  was  spread  over  the 
upper  layer,  care  being  taken  in  each  layer  to  leave 
quite  I  in.  of  the  castings  projecting  outwardly,  free 
from  cement.  A  second  iron  ring,  precisely  similar  to 
the  first,  was  now  laid  over  the  top,  and  when  the 
cement  had  set,  clamped  to  the  lower  one  by  means  of 


.three  long  screw  clamps.  The  iron  strip  attached  to  the 
inner  end  of  one  casting  was  then  soldered  to  the  iron 
strip  attached  to  the  outer  end  of  its  neighbour  (previously 
cut  to  the  required  length),  and  so  on  all  round  each 
circle,  except  at  one  point  of  each  circle  only,  where,  of 


14$  ELECTRICAL  INSTRUMENT  MAKING  FOR  AHA  TEUR3. 

course,  the  projecting  strips  were  left  free,  to  be  afterwards 
attached  to  binding  screws  as  terminals.  Each  circle  had 
its  own  pair  of  terminals,  and  these  could  be  coupled  up 
to  the  neighbouring  circles  either  in  parallel  or  in  series^ 
as  the  case  might  demand.  This  pile  was  used  with  a 
large  paraffin  burner  having  an  iron  chimney  nearly 
couching  the  interior  ends  of  elements.  It  has  been 
also  tried  with  a  charcoal  brazier  (with  a  similar  iron 
chimney),  with  gas,  and  with  a  small  coke  stove.  For 
steady  currents  of  lowE,M.F.,  but  considerable  quantity, 
the  6  in.  paraffin  burner  answered  admirably. 

The  following  table  will  give  a  fair  idea  of  the  force 
developed  with  the  said  paraffin  burner,  the  elements 
being  in  all  ttjese  cases  coupled  up  for  tension — *>.,  in 
scries  :— 

No.  of  Amperes  on  8 

Elements.  E.M.F.  short  circuit. 

36  2  0*6 

72  4  0-6 

'108  6  o-6 

144  8  0-6 

•p1gr~"56  is  reproduced  from 'a  photograph  of  the 
identical  thermopile  (of  four  circles  of  36'  elements) 
with  which  the  above  trials  were  made.  By  using 
more  powerful  sources  of  heat  (up  to  a  certain  point) 
correspondingly  more  powerful  effects  were  obtained. 

S  95.  BATTERIES. — For  the  convenience  of  classifica- 
tion, batteries  may  be  divided  into  two  great  families— 
viz.,  single  fluid  and  double  fluid.  To  the  former  class 
belong  all  such  as  do  not  require  a  partition  of  any  kind 


BATTERIES.  147 

(whether  porous  cell,  septa,  sa\vdust,  sand,  difference  of 
specific  gravity,  etc.)  betv/een  the  fluid  surrounding  the 
negative  and  the  positive  plate  or  element 

To  the  latter  class  belong  all  those,  in  which,  either 
for  the  sake  of  obtaining  constancy  of  effect,  overcoming 
polarisation,  etc.,  the  fluid  or  fluids  surrounding  the 
negative  and  positive  elements  respectively,  are  kept 
from  mixing,  by  any  of  the  means  just  mentioned. 

Few  amateurs  would  care  to  manufacture  their  o'vn 
jars,  or  porous  cells,  so  no  attempt  will  be  made  here  to 
describe  the  construction  of  such,  except  to  point  out 
that  where  great  lightness  and  strength  is  required,  as 
in  the  case  of  small  batteries  to  drive  model  yacht  motor, 
or  pocket  coils,  very  efficient  and  perfectly  acid  proof 
cells,  of  any  shape,  may  be  constructed  by  gluing  to- 
gether with  good  tape,  stout  brown  pasteboard,  of  the 
size  and  form  required.  The  cells  thus  formed,  after 
being  allowed  to  dry  thoroughly,  must  be  immersed  fot 
a  few  minutes  in  hot  melted  paraffin  wax  until  thoroughly 
permeated,  and  then  allov/ed  to  dry  and  set.  Cells  of 
this  kind  will  stand  any  acid,  and  even  a  solution  of  sul- 
phate of  copper. 

Small  porous  cells  may  be  made  out  of  bowls  of  to- 
bacco pipes,  the  small  hole  being  stopped  with  Prout's 
elastic  glue.  Larger  ones  for  any  experimental  purposes, 
had  much  better  be  bought,  but  can  be  made  by  the 
amateur  from  any  good  clean  yellow  clay  kneaded  so  as 
to  free  it  from  stones,  etc.  This  may  be  moulded  of  the 
desired  shape,  allo\yed  to  dry  perfectly  and  then  gradually 
heated  to  redness  in  any  ordinary  fire.  Greater  porosity 


148  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS, 

may  be  imparted  to  the  clay  by  the  addition  of  powdered 
graphite  or  even  charcoal. 

Zincs  may  be  cut  to  any  shape  by  making  a  pretty 
deep  line  with  a  file  at  the  spot  where  it  is  desired  to 
divide,  and  then  running  a  little  quicksilver  in  the  furrow 
thus  produced.  In  a  few  seconds  this  permeates  through 
the  zinc  at  this  place,  rendering  it  brittle  and  rotten,  so 
that  the  least  pressure  suffices  to  cause  the  zinc  to  break 
at  the  line. 

Amalgamation  is  best  effected  by  making  up  a  mix- 
ture of  I  part  oil  of  vitriol  with  19  parts  of  water,  placing 
this  in  a  large  flat  shallow  dish,  in  which  a  little  mercury 
is  also  placed.  The  fingers  having  been  rubbed  with  a 
greasy  rag  to:  prevent  the  acid  affecting  the  skin,  the 
zinc  plates  or  rods  are  one  by  ofie  immersed  in  the  acid 
and  quickly  rubbed  over  with  an  old  tooth  brush  so  as 
to  carry  the  mercury  all  over  the  surface.  The  excess 
of  mercury  should  be  allowed  to  drain  off  by  rearing  the 
plates  on  end,  in  a  plate  or  other  earthenware  vessel. 
In  separating  the  positive  and  negative  elements  from 
each  other,  ebonite  will  be  found  of  the  highest  value  for 
small  batteries.  In  larger  ones,  teak,  mahogany,  or  box- 
wood strips,  previously  boiled  in  hot  melted  paraffin  wax, 
give  excellent  results,  and  are  impermeable  to  the  acid 
used. 

Binding' screws y  though '  very  convenient,  are  not 
absolute  necessities.  In  many  cases  the  negative  plates 
(even  if  of  graphite)  can  be  held  in  their  places  by 
ordinary  wood  screws,  by  being  screwed  to  the  wooden 
bar  which  separates  them  from  the  zinc,  Connecringr 


TEE  SINGLE  FLUID  CELL.  149 

.wires  can  be  soldered  to  the  zinc,  or  twisted  tightly 
found  the  shoulders  of  the  screws.  For  coupling  up  a 
number  of  elements  or  circuits,  strips  of  copper,  about 
I  in.  wide,  i\  in.  long,  about  sfo  in.  thick,  made  perfectly 
clean  and  bright,  and  then  rolled  contrariwise  at  each 
end,  so  as  to  present  the  aspect  (in  section)  of  an  CO ,  will 
be  found  very  convenient.  It  is  not  proposed  here  to 
give  details  for  the  construction  of  all  the  batteries 
which  have  been  from  time  to  time  "  invented,"  patented, 
or  described,  for  their  name  is  legion,  and  their  utility 
in  many  cases,  highly  problematic.  General  outlines 
will  be  given  for  the  construction  of  a  single  fluid,  and 
double  fluid  battery ;  the  student  can  then  use  what 
excitant  he  may  fancy,  or  circumstances  dictate. 

§  96.  THE  SINGLE  FLUID  CELL. — For  the  contain- 
ing vessel,  a  Westall  salt  jar,  or  one  of  the  2  Ib.  plum 
bottles,  wiU  do  very  well.  For  very  small  cells,  the 
s  mailer  sizes  of  Liebig's  "  extract  of  beef  "  pots,  answer 
admirably.  The  zinc  and  copper  (or  graphite)  elements 
having  been  cut  of  the  right  size  to  enter  the  vessel,  and 
yet  leave  a  good  £  in.  clear  between  the  zinc  and  nega- 
tive element,  two  strips  of  paraffined  wood,  £  in.  thick, 
and  from  \  in.  to  I  in.  wide,  according  to  the  size  of  the 
battery,  are  cut,  a  little  longer  than  the  diameter  of  the 
containing  cell.  A  small  strip  of  clean  sheet  copper,  to 
one  end  of  which  the  wire  which  is  to  form  one  pole  of 
of  the  battery  is  soldered,  is  placed  in  contact  with  the 
upper  end  of  the  zinc  plate.  Over  this  is  to  be  placed 
one  of  the  paraffined  wooden  strips :  then  the  negative 
element.  If  the  amateur  is  content  with  a  single 


ISO   ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 

negative  element  (be  it  copper,  graphite,  silver,  or 
platinum),  he  need  now  only  place  the  second  strip  of 
paraffined  wood,  over  the  top  edge  of  the  negative  plate 
with  a  little  copper  strip  and  wire  (  as  before,  to  form 
the  other  pole  of  the  battery)  between  the  negative 
plate  and  wooden  strip.  The  whole  is  now  clamped 
together  between  the  jaws  of  a  clamping  binding  screw, 
or,  if  the  larger  elements  are  used,  between  the  jaws  of 
a  sewing  machine  clamp  (these  can  be  got  at  id.  and 
lid.  each).-  -Care  must  be  taken,  that  the  binding  screw 
or  clamp,  does  not  make  contact  between  the  negative  and 
positive  elements.  As  much  better  results'  are  obtained 
when  the  negative  elements  are  double,  the  amateur  will 
probably  prefer  to  have  two»  carbons,  or  coppers,  to  each 
zinc.  In  this  case,  after  having  placed  the  copper  piece, 
to  which  the  wire  is  attached,  against  the  top  end  of  the 
zinc,  he  will  put  a  paraffined  wooden1  strip  on  each  side 
of  the  top  end  of  the  zinc,  then  a  negative  element  on 
each  side  of  the  zinc.  If  the  negatives  are  graphites,  a 
wide  copper  band  should '  encircle  both ,  graphites,  and 
the  c1  amp  should  grip  the  band  against  the  graphites ; 
care  being  taken  as  before,  that  the  graphites,  neither 
through  the  clamp,  nor  through  the  copper  band  at  any 
place  make  contact  with  the  zinc. 

§  97.'  MOUNTING  GRAPHITE  RODS  AND  PLATES.— 
In  cases  where  the  negative  elements  are  graphite,  and 
more  especially  if  the  plates  are  to  stand  long  in  the 
fluid,  as  in  the  Leclanche,  the  bottle  bichromate,  etc.,  it 
is  advisable,  owing  to  the  porosity  of  the  graphite,  to 
adopt  some  other  means  of  making  connection  with  the 


MOUNTING  GRAPHITE  RODS  AND  PLATES.          151 

terminal.  It  is  usually  recommended  to  electroplate  the 
upper  end  of  the  carbon  with  copper,  and  then  solder 
connection  to  it,  but  a  better  plan,  is  to  make  several 
nicks  round  the  plate  or  rod,  with  a  file,  or  even  to  drill 
a  few  holes  in  with  a  screw  drill,  and  then  to  cast  a 
leaden  cap  round  the  top  end.  If  the  lead  have  a  lit'tle 
antimony  added  to  it  during  fusion,  it  will  set  much 
harder,  and  fit  closer.  This  is  hardly  acted  on  by  the 
usual  acids  (  of  the  battery.  To  prevent  any  chance 
access  of  acid,  the  top  end  of  the  -graphite  may  be 
painted  round  with  hot  paraffin  wax.  This  treatment  is 
specially  useful  in  the  case  of  bichromate  or  chromic 
acid  cells. 

§  98.  It  is  a  well-known  fact  that  the  remarkable  fall 
in  current  strength  which  takes  place  in  the  single  fluid 
batteries  of  this  class  depends  to  a  great  extent  on  the 
absolute  immobility  of  the  exciting  fluid.  Thi ;  is  due 
to  the  fact  that  as  the  hydrogen  is  absorbed  by  the 
oxidant  (chromic  acid,  bichromate  of  potash)  as  fast  as 
it  is  generated,  no  mechanical  movement  is  produced 
in  the  mass  of  the  liquid,  so  that  the  liquid 
near  the  zinc  plate  becomes  quickly  charged  with 
sulphate  of  zinc,  thus  protecting  the  plate  from  the 
farther  action  of  the  acid.  Many  schemes  have  been 
proposed  to  avoid  this,  such  as  setting  up  circulatory 
currents  in  the  fluid  by  the  external  application  of  heat 
(Sprague),  or  by  means  of  an  aspirator  (Courtenay),  etc., 
etc.  These  methods  are  excellent  in  their  way,  but  are 
rather  inconvenient  of  application  by  the  amateur,  who 
has  only  to  deal  with  a  few  cells — and  fewer  shillings. 


152  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEVRS. 

A  very  simple  mode,  which  is  quite  effective,  which 
gives  no  more  trouble  than  snuffing  a  candle,  and  which 
could  be  made  automatic  if  desired,  is  one  which  the 
author  adopted  in  his  own  4-coll  batteries  for  temporary 
lighting  purposes,  etc. 

The  battery  (shown  below,  Fig.  5)  consists  essentially 
of  foitfjjlass  cells, -A  A  A  A,  about  2\  in.  in  diameter, 


Fio.  57. 

standing  on  a  tray,  T,  from  the  centre  of  which 
rises  a  screwed  and  jointed  rod,  R,  by  means  of  which 
it  can  be  raised  or  lowered,  along  with  the  four 
cells  in  the  box  B.  These  cells  are  filled  to  about 
two-thirds  of  their  height  with  the  excitant  (chromic 
acid  3  parts,  sulphuric  acid  3  parts,  water  17  parts). 
The  zincs  and  carbons  are  attached,  by  means  of  long 


THE  DOUBLE  FLUID  CELL.  153 

binding- screws,  to  the  lid  L,  and  each  element  is 
connected  in  series  to  its  neighbour  by  means  of  metal 
straps,  the  first  and  last,  of  course,  forming  the  electrodes. 
The  rod  R,  passing  through  the  lid,  enables  the  operator 
to  raise  the  cells  to  the  plates  ;  and  this  in  practice  will  be 
found  a  great  advantage  over  lowering  the  plates  into 
the  cells.  The  arrangement  for  setting,  up  movement 
in  the  fluid  consists  simply  in  ebonite  rings,  E  E  E  E, 
which  encircle  the  plates,  and  which -are  attached  to  the 
ends  of  guttapercha-covered  wires,  \v\v  w  W,  the  upper 
extremities  of  which  pass  through  the  lid  of  the  box,  and 
are  soldered  to  the  four  corners  of  a  flat  square  of  wire, 
F,  which,  on  being  raised  and  depressed,  agitates  the 
fluid  in  the  cells,  and  thus  prevents  the  accumulation  of 
zinc  sulphate  round  the  plates.  For  the  convenience  of 
carriage  etc.,  the  lid  L  is^  fastened  to  the  box  by  means 
of  two  rings  and  catches,  and  is  furnished_with_a,central 
handle,  not  shown  in  the  sketch.*^ 

§  99.  THE  DOUBLE  FLUID  CELL.— Differs  from" the 
single  fluid  cell  only  inasmuch  as  a  porous  cell,  or 
substitute,  is  employed  to  separate  thq  fluid  acting  on 
the  zinc  from  that  in  contact  with  the  negative  element. 

As  before,  the  containing  vessel  may  be  a  stoneware 
jar  or  glass  wide-mouthed  bottled  A  porous  cell  is 
chosen,  a  little  taller  than  the  containing  or  outer  cell, 
^he  bottom  of  this  porous  cell,  and  also  £  in.  round  the 
top,  should  be  immersed  in  hot  melted  paraffin  wax. 
This  prevents  "creeping"  and  is  specially  serviceable 
in  the  case  of  the  Daniell  cell.  The  zinc  may  take  the 
form  of  a  casfrod,  and  this  obviates  the  necessity  of  a 


154   ELECTRICAL  INSTRUMENT UAK1NG FOR  AMATEURS. 

binding  screw,  as  a  copper  wire  may  be  cast  in  as  a 
terminal.  At  one  time  "  cast  "  zinc  was  regarded  with 
suspicion  ;  but  now  it  has  been  pretty  well  proved  that 
cast  zinc,  even  if  it  contain  a  certain  amount  of  tip  and 
lead,  is  quite  as  efficient,  if  not  even  more  so  than 
the  rolled  metal.  But  in  any  case  it  must  be  well 
amalgamated.  A  little  vooden  lid  or  cover  should  be 
fitted  to  the  porous  cell,  through  the  centre  of  which 
should  project  the  wire  coming  from  the  zinc.  The 
negative  element,  if  copper,  may  be  bent  into  the  form 
of  a  circle  to  fit  the  inside  of  the  jar,  and  the  other 
terminal  soldered  to  it.  Also  a  little  shelf  or  ledge 
should  be  soldered  inside  Hie  coppe/,  at  the  top  edge,  to 
hold  crystals  of  copper  sulphate,  if  the  Daniell  form  be 
preferred.  If  carbon  be  chosen  as  the  negative,  then  it 
will  be-  well  to  take  a  sufficient  number  of  carbon 
(graphite)  pencils,  such  as  are  used  for  electric  lights, 
about  i  in.  thick  and  a  little  longer  than  the  outer  cell, 
and  having  tied  them  all  round  a  bottle  or  other 
cylindrical  body,  cf  such  a  size  that  they  will  then  freely 
enter  the  outer  vessel,  cast  a  leaden  ring  round  one 
extremity,  to  which  the  other  electrode  can  be  attached. 
This  forms  a  very  excellent  negative,  presenting  a  very 
large  surface.  Annexed  is  a  table  of-  the  names, 
elements,  fluids,  and  E.M.F.,  etc.,  of  the  most  useful 
batteries ;— 


THE  DOUBLE  FLUID   CELL. 


155 


NAME  OF 
CELL. 

POSITIVE 

ELEMENT. 

NEGATIVE 
ELEMENT. 

EXCITING 
FLUID: 

DEPOLARIS- 
ING FLUID. 

E.M.F.  ra 
VOLTS. 

INTERNAL 
RESISTANCE 
IN  OHMS.* 

Bunscn 

Zinc 

Graphite 

Sulphuric 
Acid  dilute 

Nitric  acid 

i'734 

•08  to  -ii 

Do. 

" 

M 

" 

Chromic 
acid 

»'734 

'1  tO  '13 

Chromic 
Acid,  single 
fluid 

• 

- 

Sulphuric 
acid  and 
chromic 
acid,  dilute 

None 
separate 

3-3 

•001  to  -08 

Daniell 

" 

Copper 

Zinc  sul- 
phate solutn 

Copper  sul- 
phate sol. 

1-079 

2-  to  5 

Fuller 

» 

Graphite 

Chloride  of 
zinc 
solution 

Potash  bi- 
chromate 
and  hydro- 
chloric acid 

*'S 

0-5  to  0-7 

Gaiffe 

" 

Silver 

Zinc 
chloride 

Silver 
chloride 

I  '02 

0-5  to  06 

Grove 

* 

Platinum 

Sulphuric 
acid  dilute 

Nitric  acid 

I-96 

'I  tO  '13 

Lalande 
Chaperon 

" 

Copper  or 
iron 

Caustic  pot- 
ash sjlution 

Oxide  of 
copper 

0-98 

1-30 

Latimer 
Clark 

" 

Pure 
mercury 

Sulphate  of 
mercury 

None 
separate 

''457 

o'j  to  o's 

Leclanche" 

" 

Graphite 

Ammonium 
chloride  sol. 

Manganese 
dioxide 

r48 

1-13  to  1-15 

Maiche 

Zinc  scraps, 
in  bath  of 
mercury 

Platinized 
carbon 

Common  salt 
solution 

None 
separate 

i'2S 

i-  to  'a 

Marie  Davy 

Zinc 

Graphite 

Sulphuric 
acid  dimte 

Paste  of 
sulphate  of 
mercury 

»'524 

•75  to  i' 

Niaudet 

" 

" 

Common  sait 
solution 

Chloride 
of  lime 

j-5  to  1-6- 

5  to6' 

Poggendcrf 

» 

- 

Saturated 
sol  of  p  -tash. 
bichromate, 
and  sul- 
phuric acid 

None 
separate 

1-98 

•ooi  to  -08 

Schansch.eff 

" 

" 

Mercurial 
solution 

None 
separate 

i-56 

•05  to  0-75 

Skrivanow 

" 

Silver 

Caustic 
potash 

Chloride  of 
silver 

i'S 

i'S 

Smee 

» 

Platinized 
silver 

Sulphuric 
acid  dilute 

None 

o-47toi-i 

°'S 

Walk  r 

" 

Platinized 
graphite 

" 

•' 

o-ee 

o'4 

Warren  de  la 
Hue 

" 

Silver 

Sal  ammo- 
niac solution 

Silver 
chloride 

i* 

0.4  to  o'6 

:  The  resistances  were  measured  in  cells  standing  6"  X  4' 


156  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMA  TEURS. 

§  ioo.  THE  TELEPHONE.— Although  the  effects  pro- 
duced by  this  instrument  are  at  once  among  the  most 
beautiful  and  astounding  in  the  whole  rahge  of  physics, 
nevertheless  the  apparatus  necessary  to  their  production 
is  of  the  simplest  description.  In  the  form  patented  by 
Graham  Bell  (see  fig.  60),  which  embodies  all  the  essen- 
tial points  of  a  serviceable  working  instrument,  we  have 
a  bar-magnet,  around  one  pole  of  which  is  coiled  about 
a  hundred  feet  of  fine  insulated  copper  wire.  The  ex- 
tremities of  this  coil  of  wire  are  attached  to  two  binding 
screws,  by  means  of  which  connection  can  be  made  to 
the  transmitting  lines,  etc.  In  front  of  the  coiled  pole 
of  the  bar-magne't,  but  not  in  actual  contact,  is  a 
circular  plate  of  very  thin  sheet  iron,  gripped  at  its 
edges,  but  free  to  vibrate  centrally.  This  arrangement 
is  ail  that  is  really  necessary  in  the  construction  of  the 
telephone.  In  order  to  carry  on  a  conversation  by  the 
aid  of  the  arrangement  described,  two  precisely  similar 
instruments  are  employed,  one  at  the  speaking  or 
"transmitting"  end,  and  the  other  at  the  hearing  or 
"  receiving  "  end.  The  two  binding  screws  belonging  to 
each  instrument  are  connected  together  by  means  of 
separate  insulated  wires.  An  individual  speaking  near 
the  thin  iron  disc  of  one  of  the  telephones,  causes  the 
air  to  enter  into  vibration.  These  vibrations  are  taken 
up  by  the  thin  iron  disc,  which  performs,  in  consequence, 
oscillations,  "  excursion  and  incursions "  to  and  from 
the  pole  of  the  magnet.  By  virtue  of  the  well-known 
law,  that  "  motion  before  the  poles  of  a  magnet  sets 
up  currents  of  electricity  "  (see  §  58),  and  that  the 


THE  TELEPHONE.  157 

Currents  flow  in  one  direction  when  the  movement  is 
one  of  approach,  and  in  the  contrary  when  the  motion 
is  one  of  recession,  it  follows  that  during  speech  a 
number  of  waves  of  electricity,  now  in  one  direction, 
now  in  another,  flow  around  the  wire  encircling  the 
coiled  pole  of  the  magnet,  and,  traversing  the  lines 
leading  to  the  farther  instrument,  flow  round  the  coiled 
pole  of  that  magnet.  When  these  flow  in  one  direction, 
the  magnet  is  strengthened  by  their  advent,  and  pulls 
down  the  disc  before  it  more  forcibly.  When  the  flow 
is  in  the  opposite  direction,J:he  magnet  is  correspond- 
ingly weakened,  its  pull  on  the  disc  is  lessened,  and 
consequently  the  disc  performs  an  excursion  from  the 
magnet  by  virtue  of  its  elasticity ;  and  every  trifling 
modification  in  the  strength  and  direction  of  current 
set  up  by  the  vibrations  in  the  disc  moved  by  the 
speaker's  voice  at  the  transmitting  end,  is  faithfully 
reproduced  in  vibrations  of  corresponding  amplitude 
and  strength,  in  the  disc  at  the  receiving  or  hearing  end. 

To  construct  a  pair  of  telephones  for  experimental 
purposes,  capable  of  transmitting  to  a  distance  of  two 
or  three  miles,  the  following  materials  will  be  needed  : 

1st.  A  pair  .of  square  bar-magnets,  about  \  in.  by  3  in. 
such  as  may  be  procured  from  the  ironmongers  from 
l£d.  01;  2d.  each.  These  should  be  capable  of  sustaining 
eacho':her  if  the  marked  end  of  the  one  be  presented 
to  the  unmarked  end  of  the  other. 

2nd.  A  pair  of  wooden  tooth-powder  boxes,  2^in. 
diameter  by  about  I  Jin.  deep.  These  may  be  obtained 
at  any  chemist's  shop. 


158  ELECTRICAL  INSTRUMENT  MAKING  FOP  AMATEURS. 

3rd.  Axpair  of  ferrotype  plates,  4^  in.  by  3^  in.  These 
-are  to  be  bought  from  the  dealers  in  photographic 
goods,  being  the  thin  iron  plates  on  which  the  cheap 
"  ferrotype  "  pictures  are  taken. 

T4th.  Two  pairs  of  small  binding  screws/  of  any 
pattern,  so  long  as  they  are  small. 

5th.  A  piece  of  cylindrical  white  wood,  similar  to  a 
broom -handle,  about  I  in.  dr  meter,  and  12  in.  long. 

6th.  A  quarter  of  an  ounce  of  No.  36  silk  covered 
copper  wire. 

§  101.  The  operator  begins  by  cutting  off  two  pieces  of 
the  cylindrical  wooden  rod,  4^  in.  in  length,  and,  with  a 
sharp  knife,  splits  each  one  down  the  middle,  into  two 
semi-cylindrical  halves.  Taking  care  to  keep  each 
pair  of  halves  so  as  tovbe  able  to  fit  them  together 
again  when  required,  he  cuts,  with  a  £-in.  chisel,  a 
channel  in  the  flat  face  of  each  half-cylinder,  to  such  a 
depth  that  the  bar-magnets  can  lie  between  the  two 
halves  when  the'se  are  fitted  together.  The  channels 
should  extend  from  end  to  end  of  the  cylinders,  and 
should  just  allow  the  magnets  to  slide  in  them,  without 
any  lateral  play.  At  one  end,  for  a  depth  of  about 
2  in.,  the  channels  should  be  made  a  trifle  wider  than 
the  bar-magnets,  say  ^  in.  When  the  channels  have 
been  thus  satisfactorily  cut,  the  half  cylinders  must  be 
glued  together,  so  as  to  form  two  cylinders  with  a 
square  channel  running  up  the  middle.  The  next  step 
consists  in  cutting  a  circular  hole  (of  exactly  the  same 
diameter  as  the  wooden  cylinders  just  prepared)  in  the 
centre  of  the  bottom  of  each  _  tooth  -powder  box.  A 


THE  TELEPHOKn.  159 

little  good  glue  is  now  rubbed  round  the  edges  of  this 
hole,    and   also   round  the  top    end    of  each    cylinder. 
(The   top    end    is    the    end    at    which    the    channel  is 
smallest).     This  end  is  then  thrust  into  the  bottom  of 
the  box,  from  the  outside,  until  it  is  just  flush  with  the 
inside  of  the  bottom  of  the  box.     The  boxes,  with  their 
cylindrical  handles,  are   now  set  aside  to  dry.     Whilst 
these  are  drying,  the  student  may  cut  a  circular  aperture 
l|  in.  in  diameter,  in  the  centre  of  the  lid  of  each  tooth- 
powder  box.     He  then  proceeds  to  glue  a  cone  of  stiff 
pasteboard,     shaped    something    like    the    bell    of    a 
clarionet,  into  this  hole.     This  cone,  or  bell,  must  open 
and    extend    outwards    for    about    I   in.,   and    be    cue 
off  flush   with  the   inside  of  the  lid  of   the  box.     Its 
only   service  is    to  collect  and   re-enforce    the    sounds 
which    are    to    fall    on  the  vibrating  plate.     The  next 
step  is  to  cut    two   circles  out  of  the  ferrotype    plates 
of  such  a  diameter  as   to  fit   exactly,  without  binding, 
or  without  any  shake,  into  the  inside  of  the  lids   of  the 
boxes.      As    any   dent   or   buckling    in    the    ferrotype 
plates  would  prove  fatal  to  their  action  in  the  telephone, 
it  is  rtot  permissible  to  use  a  compass,  or  any  similar 
instrument,  wherewith  to  strike  the  circles  on  them;  but 
the  following  mode  of   procedure    must    be    adopted  : 
Having   measured  the  exact  diameter  of  the  inside  of 
the  lids,  the   operator  strikes  out,  with  the  compasses,  a 
corresponding   circle  on    a  stout  piece    of  cardboard  ; 
cuts  out  this  circlet  with   the  scissors,  and  then,  laying 
this  vas   a  template  over   the   ferrotype  plate,  scratches 
lightly  a  line  all  round  with  the  point  of  a  pin.     It  is 


iCo  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMA  TEURS'^ 

easy  then,  with  a  sharp  pair  of  scissors/"  to  cut  a1 
circle  of  the  desired  size  in  the  ferrotype^  plates,  ^without 
any  buckling  or  denting. 

Two  pieces  of  brass  wire,  about  £  in/  in  diameter  and 
2  in.  long,  are  now  run  through  a  screw  plate,  so  as  to 
produce  a  thread  on  them  for  their  entire  length.  Two 
small  hexagonal  nuts  are  cut  out  of  a  piece  of  sheet  brass 
\  in.  thick,  and  a  corresponding  female  screw  produced 
in  them. 

A  piece  of  brown  paper,  \  in.  wide,  is  glued  around  one 
end  of  these  screwed  wires,  until  this  end  is  of  the  same 
thickness  as  the  sealing-waxed  end  of  the  bar-magnets. 
When  this  is  the  case,  the  screw  wires  are  each  respec- 
tively glued,  as  prolongation,  to  the  sealing-waxed  ends 
of  the  bar-magnets,  one  to  each,  and  further  strengthened 
in  their  attachment  by  having  a  roll  or  two  of  brown 
paper  glued  tightly  round,  of  such  a  length  as  to  embrace' 
about  fin.  up  the  bar  magnet  and  £in.  of  the  screw.  The 
thickness  of  this  paper  must  not,  however,  exceed  the 
width  of  the  channel  at  the  lower  end  of  the  cylinder. 

A  small  bobbin  of  cardboard  is  now  made  to  fit  the 
polished  ends  of  the  bar-magnets.  These  bobbins  should 
be  about  I  in.  in  diameter  by  about  I  in.  wide  in  the 
channels  ;  they  should  be  made  to  fit  pretty  tightly  on  to 
the  poles  of  the  magnets.  When  made  and  glued 
together,  they  should  be  allowed  to  dry,  and  then  soaked 
for  a  minute,  or  two  in  melted  paraffin  wax.  After  this 
they  may  be  wound  with  about  I  drachm  (60  grains)  of 
No.  36  silk-covered  copper  wire,  particular  care  being 
taken  to  wind  in  one  continuous  direction  only,  and  to 


THE  TELEPHONE.  ifi 

avoid  all  breakages  or  even  kinks  in  the  wire,  as  being 
fatal  to  success.  Each  bobbin,  after  being  wound  (and  it 
should  be  wound  while  on  the  pole  of  the  magnet),  should 
be  removed  from  off  the  pole  of  the  magnet  and  dipped 
for  an  instant  in  melted  paraffin  wax.  About  3  in.  of 
each  end  of  the  wires  should  be  left  free,  for  attachment 
to  the  binding  screws. 

This  being  done,  the  nut  is  removed  from  the  screwed 
tailpiece  of  each  magnet.  Two  small  circlets  are  cut 
out  of  thin  sheet  brass  or  zinc,  of  the  same  diameter  as 
the  lower  ends  of  the  cylinders.  A  central  hole  Is  bored 
in  these  circlets,  to  admit  of  the  passage  of  the  screwed 
tailpieces  of  the  magnets  ;  and  two  lateral  holes,  by  means 
of  which  they  can  be  fastened  to  the  ends  of  the  cylinders 
with  two  screws.  The  bar-magnets  are  then  pushed  up 
the  central  channels  until  their  polished  ends  are  very 
nearly  flush  with  the  edges  of  the  boxes,  before  the  lids 
are  on.  The  little  brass  circlets  just  prepared  are  then 
screwed  on  to  the  ends  of  the  cylinders,  leaving  about 
\  in.  of  the  screwed  tailpieces  projecting.  (Should  the 
magnets  play  too  loosely  in  the  channels,  a  thin  sheet  of 
paper  maybe  wrapped  round  them,  to  increase  the  fric- 
tion). The  nuts  may  now  be  replaced  on  the  tailpieces 
Each  wound  bobbin  is  now  to  be  slipped  in  its  place 
over  the  pole  of  its  respective  magnet,  and,  if  too  loose, 
retained  there  by  touching  the  inside  of  the  bobbin  with 
a  brush  lightly  dipped  in  white  hard  varnish.  The  free 
ends  of  the  wire  proceeding  from  each  bobbin  are  soldered 
to  the  prongs  of  the  binding  screws,  which  are  driven  into 
the  bottom  of  the  boxes,  at  opposite  points  in  its  diameter, 


162  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 

near  the  edges.     The  ferrotype  plates  are  now  placed 


inside  the  lids,  with  the  glossy  side  outwards,  and  the 
dids  pressed  firmly  on  to  the  boxes,     The  lids  should 


THE  TELEPHONE.  l6j 

fit  tightly  ;  if  they  do  not,  they  should  be  made  to  do  so 
by  gluing  a  strip  of  paper  round  the  edge  of  the  rim  of 
the  box.  The  boxes,  handles,  and  cardboard  bells  may 
be  stained  and  varnished  if  desired. 

The  distance  of  the  magnets  from  the  ferrotype  plates 
can  be  regulated  to  a  nicety  by  means  of  the  nuts  and 
screwed  tailpieces  of  the  magnets.  In  practice,  I  find 
that  the  best  effect  is  obtained  when  the  magnets  are 
only  just  clear  of  the  plates.  To  secure  this  result,  it  is 
only  necessary  to  screw  up  until  the  magnet  just  touches 
the  plate.  This  can  be  told  by  the  dull  blocked  sound 
which -is  given  on  tapping  lightly  the  ferrotype  plate 
with  the  finger  nail ;  then,  if  the  nut  be  turned  gradually 
in  the  opposite  direction  until  the  tapping  gives  a  clear 
sound  instead  of  a  dull  thud,  the  magnet  will  be  just  clear 
of  the  diaphragm.  The  annexed  cut  will  make  the 
general  disposition  of  the  parts  clear:  A  is  the  bar 
magnet,  attached  by  means  of  the  brown  paper  roll  to  tho 
regulating  screw  H ;  this  is  controlled  by  the  nut  I 
which  is  rotated  against  the  circlet  J.  The  coiled  bobbin 
is  shown  at  C,  with  its  free  ends  of  wire  attached  by  a 
drop  of  solder  to  the  binding  screws  G  G.  At  F  F,  we 
have  the  channelled  cylinder,  in  which  lies  the  magnet. 
D  D.,  represents  the  tooth  powder  box,  which  forms  the 
case  and  resonant  box  ;  B  is  the  ferrotype  plate,  and 
E  E  tne  conical  mouthpiece  made  of  stout  cardboard. 


164  ELECTRICAL  INSTRUMENT  WAKING  FOR  AMATEURS. 


$  102.    ELECTRO-MOTORS. 

A  very  efficient  little  motor  for  driving  model  boats 
!or  locomotives,  and  for  producing  rotation  of  vacuum 
tubes.etc.,  may  be  constructed  in  the  following  manner : 
Procure  a  piece  of  soft  hoop  iron  about  \  in.  thick,  \  in. 
wide,  and  I  ft.  long;  cut  this  into  two  equal  pieces  6 in. 
in  length  ;  drill  a  t^in.  hole  through  the  centre  of  each 
one  ;  then  bend  each  piece  of  the  iron  into  the  shape 
of  the  letter    U,  having  the    limbs    if  in.    apart,    and 
therefore  about    2\  in.  in  length    from    the   middle    of 
bend  to  the  end  of  limb.     In  one  of  these  pieces,  which 
is  to  be  the   fixed   magnet,  two  other  holes,  in  a  line 
with  the  first,  but  near  the  edges  of  the  iron,  should  be 
drilled,  in  order  to  enable  the  operator  to  fasten  it  to 
the    base    board    when    finished.        A    piece    of    sound 
mahogany  or  well-seasoned   pine  about  3  in.  wide,  $  in. 
thick,  and   loin.  long,-should  now  be  procured;  and  a 
'piece  about  3  in.  square  cut  off  one  end  and  glued  and 
screwed    to   the   remaining   7  in.    at   right  angles   to  it, 
'similar  to  a  letter  L.     This  piece,  which  forms  the  base 
board,    may  be  stained,  varnished,  or  polished,  at  the 
option  of  the  maker.     The  next  step  is  to  wind  the  two 
U-shaped   electro-magnets  previously  produced  ;  great 
care,  of  course,  being  taken    to  cover  the  iron  first  with 
ja  dressing  of  paper  or   tape,   as   described  at  H  67  and 
'74,  to  insure  that  no  electricity  shall  leak  through  from 
the   wire,    with  which   the  magnets  are  wound,  to  the' 
iron  below.      Each  magnet  will   require  to  be  wound 


ELECTRO-MOTORS.  16$ 

with  6  layers  of  No.  24  silk-covered  wire ;    that  is  to 
say,  between  6  and  9  oz.  of  the  said  wire; 'the  exact 
amount  got  on  depending  on  the  skill  and  neatness  of 
the   operator.      The  connection  between  the  windings 
must  be  in  the  direction  shown  in  our  last  paragraph, 
viz.,  like  a  letter  (».     It  will.be  well,  in  order  to  avoid 
joins,  to  measure  off  one-half  of  the  wire  intended  to  be 
laid  on  one  electro-magnet,  and  without  cutting  it  off, 
from  the  remaining  half,  to  wind  one  limb  with  the  first 
half,  the  other  limb  being  wound  with  the  other  half. 
In  both  electro-magnets,  the  winding  should  be  begun 
near  the  bend;  just  at  those  portions  where  the  limbs 
of  the  U  begin  to  straighten.     If,  as  directed,  6  layers 
of  wire  are  got  on,  the  finishing,  or  free  extremities  of 
the  wires,  will  be  found  near  the  bend  of  the  U's.     At 
this  point  they  should  be  carefully  tied,  with  silk  of  the 
same  colour,  to  prevent  uncoiling.     If  the  operator  pre- 
fers appearance  to  efficiency,  nothing  further  need  be 
clone  to  the  coils ;  but  if,  on  the  contrary,  efficiency  be 
the  first  consideration,  it  will  be  well  to  soak  the  coils 
in  white  hard    varnish,  and  let  them  dry  in  a  warm 
"place.      Of  course,  in  either  case,  the  electro-magnets 
should  be  tested  for    insulation,  before  anything    else 
be  done  as  described   at  §  68.     The   next  step  is,  to 
fasten  one  of  these  electro-magnets  (the  one  with  sthe 
three  holes  drilled  at  the  bend)  to  the  upright  piece  of 
the  base  board,  with  the  limbs  parallel    to   the   base 
itself,   and    at   such  a  height   that   the  other  electro- 
.magnet  shall   be  able  to  rotate   freely   in  front   of  it 
Svithout  touching  the  base  ;    that  is  to  say  the  height 


iG6  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 

from  the  base  to  the  central  hole  of  the  fixed  electro- 
magnet must  not  be  less  than  \\  in  The  fixed  electro- 
magnet must  be  fastened  to  the  upright  of  the  base- 
board by  means  of  two  screws,  one  in  each  of  the  lateral 
holes  previously,  drilled  in  the  electro-magnet.  In  the 
central  hole  of  this  same  electro-magnet  is  passed  a 
piece  of  stout  brass  wire  about  \  in.  diameter  and  f  in. 
long,  and  which  has  been  filed  down  for  about  f  of 
its  length  to  such  a  size  that  it  can  enter  the  hole 
in  the  centre  of  the  electro-magnet,  leaving  a  head 
about  ^  m-  protruding  from  this  hole.  In  the  centre 
of  the  head  of  this  piece  of  brass  is  drilled  a  conical 
depression  by  means  of  the  Archimedean  drill.  This 
depression  is  to  form  one  of  the  bearings  in  which  the 
shaft  or  spindle  which  carries  the  movable  electro- 
magnet will  rotate.  We  will  designate  this  the  back 
bearing.  The  next  operation  consists  in  making  a  little 
pillar  about  2  in.  in  height,  from  a  piece  of  brass  wire 
about  \  in.  in  diameter.  About  Jin.  of  one  extremity 
of  this  pillar  should  be  reduced  by  filing  to  \  in.  in 
diameter,  and  a  thread  put  on  it  by  means  of  a  screw 
plate.  At  the  upper  extremity  a  hole  must  be  drilled 
and  tapped  ;  at  right  angles  to  the  length  of  the  pillar, 
and  at  such  a  height  that  when  the  pillar  is  screwed 
into  the  base-board  the  said  hole  shall  be  exactly  in  a 
line  with  the  conical  depression  in  the  back  bearing. 
This  hole  should  be  about  |in.  in  diameter,  and  be 
fitted  with  a  short  length  (about  I  in.)  of  brass  screw,  to 
serve  as  the  front  bearing.  The  face  of  this  screw  must 
also  have  a  conical  depression  drilled,  corresponding  to, 


ELECTRO-MOTORS.  167 

and  facing  the  one  in  the  back  bearing.  The  pillar 
may  now  be  screwed  into  the  centre  of  the  base-board 
at  a  distance  of  about  6\  in.  from  the  back  bearing,  with 
the  screw  which  forms  the  front  bearing  in  a  line  with 
and  perpendicular  to,  the  back  bearing.  If  necessary,  a 
small  nut  may  be  put  at  the  lower  extremity  of  the 
pillar"  screw  which  passes  into  the  base-board,  so  as  to 
insure  rigidity.  In  this  case  a  hole  must  be  counter- 
sunk at  the  under  surface  of  tlie  board,  so  as  to  allow 
the  nut  to  lie  flush  with  the  board.  A  piece  of  steel 
rod,  such  as  a  stout  knitting  needle,  is  now  procured, 
and  cut  so  as  to  be  a  little  longer  than  the  distance 
between  the  back  and  front  bearings.  This  is  to  form 
the  shaft  or  spindle  of  the  motor,  and  must  be  lowered 
in  temper  by  holding  over  the  flame  of  a  spirit  lamp  at 
its  two  extremities,  which  are  then  to  be  filed  to  fine 
conical  points,  so  as  to  run  freely  in  the  conical  depres- 
sion of  the  front  and  back  bearings.  These  points  can 
then  be  hardened  again  by  making  red-hot  and  plung- 
ing in  cold  water.  The  free  electro-magnet  is  now  to  be 
fitted  to  this  spindle.  For  this  purpose  the  spindle  is 
pushed  through  the  central  hole  at  the  back  of  the 
bend  ;  should  the  existing  hole  not  be  large  enough,  it 
must  be  rimed  out  until  the  spindle  will  just  enter. 
The  spindle  with  the  electro-magnet  on  it  is  then  placed 
between  the  bearings,  the  screw  of  the  front  bearing 
being  then  tightened  up  to  hold  the  spindle  immov- 
able. The  movable  electro-magnet  is  then  placed 
with  its  poles  facing,  but  not  touching  those  of  the  fixed 
electro -magnet,  a  piece  of  stout  cardboard,  ^in.  thick, 


168  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMA  TEURS. 

being  placed  between  the  poles  to  prevent  actual  con- 
tact.    The   magnet  being  held   firmly   in  this  position 
by  the  left   hand,   a   fik-mark   is  made  with   a   sharp 
triangular  file,  at  the  point  at  which  the  bend  of  the 
electro-magnet  touches  the  spindle.     The  spindle  with 
the  electro-magnet  is  now  removed  from  between  the 
centres,  and  the  electro-magnet  soldered  to  the  spindle 
at  the  point  just  marked  with  the   file.     In   order  to 
solder  effectually,  the  back  of  the  bend  of  the  electro- 
magnet, as   also   the  inside  of  the  bend,  must  be  filed 
perfectly  clean,  and  run  over  with  the  tinned  soldering- 
iron,  so  as  to  get  a  coating  of  solder,  before  attempting 
to  solder  the   spindle  thereto.      When  this    has   been 
done,    the    movable   electro-magnet   should    be   again 
placed  between  its  centres  to  see  whether  it  runs  freely 
before  the  poles  of  the  fixed   magnet,    without    cither 
touching  it  on  the  one  hand,  or  being  more  than  -^  in. 
away  from  it  on  the  other.     If  it  should  not  do  so,  the 
soldering  must  be  repeated  until  this  result  has  been 
attained.       A    commutator,    precisely    similar  in  form, 
but  somewhat  smaller  than  that  described  in  §  66,  is 
now  made,  and  fitted  to  the  spindle.     It  should  be  i  in. 
long,  and  \  in.  in  diameter,  the  central  hole  being  a  tight 
fit  on  the  spindle.     As  the  brass  cheeks  of  this  commu- 
tator must  be  carefully  insulated  from  each  other,  from 
the  spindle,  and  from  the  iron  of  the  electro-magnet, 
very  short  screws  must  be  used  ;  and  a  small  paraffined 
paper  washer  put  over  the  spindle,  against  the  electro- 
magnet, before  the  commutator  is  put  on  the  spindle, 
and  pushed  up  into  its  place,  which  is    close  against 


'ELECTRO-MOTORS:^ 


169 


the  outside    of    the    bend     of    the   movable   electro- 
magnet/ 

A  drop  of  white  hard  varnish,  painted  over  the 
spindle  where  the  commutator  is  finally  to  remain,  will 
prevent  the  '\  commutator  from  slipping  round.  The 
commutator  must  be  so  placed  on  the  spindle ;  that  when 
the  limbs  of  the  movable  eleclro-mag  net  are  exactly  op- 
posite the  limbs  of  the  fixed  electro-magnet >  the  slits  of  'the 
commutator  are  in  a  line  with  the  limbs ;  that  is,  they 
should  find  themselves  at  the,  two  "sides,  of.  the  j:om- 


FIG.  61. 

mutator,  and  not  above  and  below.  A  small  pulley, 
either  of  brass  or  wood,  may  be  keyed  or  soldered  to 
the  spindle,  to  serve  as  a  driving  wheel,  wherewith  to 
communicate  the  motion  wherever  desired.  The  free 
ends  of  the  movable  electro-magnet  wires  should  now  be 
cut  a  convenient  length  and  soldered,  one  to  each  cheek, 
of  the  com  mutator.  Two  L  shaped  springs  (to  serve  as 
brushes)  are  now  to  be  made  out  of  a  piece  of  very  thin 
and  springy  brass,  about  li  inches  in  length  beyond  the. 
bend,  and  \  inch  wide.  These  are  to  be  screwed  dovyn. 


170  ELECTRICAL  INSTRUMENT  MAKlNQ  FOR  AMATEURS. 

to  the  base-board  in  such  a  position  that  they  press 
squarely,  firmly,  yet  not  too  heavily  on  the  opposite 
side  of  the  commutator.  Finally,*one  of  the  ends  of  the 
wires  coming  from  the  fixed  electro-magnet  E  (see 
fig.  6 1)  is  connected  to  a  binding  screw  A.  The  other 
end  D  is  carried  in  a  groove  under  the  base-board  to  the 
screw  of  the  brush  C,  to  which  it  is  soldered,  or  other- 
wise electrically  connected.  The  other  brush  C  is  con- 
nected by  another  wire  passing  under  the  board  (and 
shown  at  B)  to  the  terminal  A.  The  spindle  carrying 
the  movable  electro-magnet  and  its  attachment  may 
now  be  put  in  its  bearings,  the  brushes  carefully  ad- 
justed, and  the  back  screw  N  screwed  up  until  the  spindle 
can  rotate  freely,  but  without  too  much  play  on  .its  two 
centres,  which  should  be  kept  oiled.  If  well  made,  this 
little  motor  will  run  well  with  a  single  Leclanchk,  better 
with  a  pint  bichromate,  or  chromic  acid  cell,  and  at  a 
furious  speed  with  the  .four  cells  described  and  figured 
at  §98. 

§  103.    THE  PHONOGRAPH. 

Although  the  phonograph  cannot  by  any  stretch  of 
the  imagination  be  called  an  electrical  instrument,  yet 
it  is  so  closely  allied  to  the  telephone  in  its  mode  of 
acting  that  a  short  description  of  the  manner  of  making 
a  simple  form  may  not  be  out  of  place  here.  The 
following  'directions  are  due  to  Mr.  Shelford  Bidwell, 
and  were  originally  published  in  the  English  Mechanic ; — 

"  The  most  important  part  is  the  cylinder.  This  in 
my  phonograph  is  a  hollow  brass  casting,  4^  in.  long  and 


TffE  PHONOGRAPH.  lit 

4^  in.  in  diameter.  It  is  mounted  upon  an  iron  spindle  f  in. 
in  diameter  and  16  in.  long,  atone  end  of  which  is  a 
winch-handle.  Upon  that  part  of  the  spindle  which 
lies  between  the  handle  and  the  cylinder  a  screw  is  cut, 
having  eight  threads  to  the  inch.  The  other  end  of  the 
spindle  is  left  plain.  The  cylinder  having  been  turned 
perfectly  true,  a  screw  is  cut  upon  its  surface  of  exactly 
the  same  pitch  as  the  screw  upon  the  spindle,  i.e.,  eight 
threads  to  the  inch.  The  depth  of  the  spiral  groove  thus 
formed  is  TV  in.,  and  its  breadth  is  T\  in.  It  is  better  to 
cut  it  square,  and  not  V  shaped.  Two  brass  bearings 
for  the  spindle  are  made  of  the  following  dimensions : — 
length,  2\  in. ;  thickness,  i£  in. ;  height,  i£  in.  One  of 
these  has  an  inside  screw  corresponding  to  the  screw 
upon  the  spindle.  Each  bearing  .has  two  holes  for 


FIG.  62. 

screwing  it  to  the  support,  as  may  be  seen  in  Fig.  62, 
which  is  engraved  from  a  photograph  of  the  instrument. 
The  cylinder,  spindle,  and  bearings  being  completed, 
ten  pieces  of  wood  must  be  prepared  as  follows  : — 

A  is  12  in.  x  9!  in.  x  i£  in. 

B  is  3  in.  x  3  in.  x  i£  in. 

C  is  similar  to  B. 


l;*  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 

D  is  5£  in.  x  5^  in.  x  \  in.,  and  has  a  circular  hole 
2f  in.  in  diameter,  cut  in  its  centre. 

E  is  similar  to  D. 

F  is  5£  in.  x  5J  in.  x  \  in.,  and  has  a  hole,  i  in.  in 
diameter,  in  its  centre. 

G  is  5£  in.  x  2^  in.  X  \  in.j 

H  is  similar  to  G. 

l  is  8  in.  x  -2\  in.  x  f  in., 

K  is  similar  to  I. 

B  and  C  are  the  upright  supports  for  the  bearings, 
^one  of  them  being  shown  in  Fig.  62.  The  position  of  the 
others  is  indicated  by, the  letters  corresponding  to  them 


FIG.  63.  FIG.  64. 

in  Figs.  63  and  64.  The  uprights  B  and  C  are  fixed  near 
the  ends  of  the  base  board,  A,  by  means  of  2\  in,  screws. 
D  and  F  are  screwed  together  (see  Fig.  63)  and  the  two 
are  then  fixed  perpendicularly  upon  G.  G  is  joined  to 
I  by»  a  pair  of  hinges.  The  two  ends  of  I  are  screwed 
to  the  base-board,  but  two  holes  in  I  are  \  in.  larger  in 
diameter  than  the  screws  which  pass  through  them. 
The  heads  of  the  screws  are  effectively  enlarged  by  iron 


THE  PHONOGRAPH  17$ 

washers  f  in.  in  outside  diameter.     The  object  in"  this 
arrangement  is  to  allow  a  certain  amount  of  play  in  the 
board  I  for  purposes  of  adjustment.     When  properly 
adjusted  the  screws  may  be  tightened,  and  the  board 
firmly  fixed  in  position.     E  is  attached  perpendicularly 
to  H,  and  made  rigid  with  two  small  triangular  pieces 
which  are  not  shown  in  the  figure.     H  is  hinged  to'  K, 
and  K  is  fixed  to  the  base  in  exactly  the  same  manner 
as  I.     Through  the  middle  of  I  is  passed  a  brass  screw 
bolt,  the  square  head  of  which  is  fixed  in  I.     The  screw 
goes  through  an  elongated  hole  in  G  and  is  fitted  with 
a  round  milled  brass  nut     It  is  well  to  plaqe  a  washer 
under  the  nut.     Screw-bolts  of  this  description  are  used 
for  fixing  the  expanding  bodies    of    ordinary  photo- 
graphic cameras,  and  may  be  had  of  any  optician.     H 
and  K  are  fitted  with  a  similar  bolt.     Two  rather  stiff 
pieces  of  steel  spring  are  attached  to  the  ends  of  I  and 
extend    for    a    little    distance    underneath    G.      These 
springs  tend  to  separate  G  and  I,  or  rather  to  cause  G 
to  turn  backwards,  like  the  lid  of  a  box  when  opened., 
One  of  the  springs  may  be  seen  in  Fig.  63. 

11  The  nut,  of  course,  works  against  the  springs. 
When  the  nut  is  screwed  up  tight  G  and  I  approach, 
and  may  be  made  almost  to  touch  each  other.  When 
the  nut  is  loosened  the  spring  causes  G  to  rise.  Very 
delicate  adjustment  is  thus  rendered  possible.  H  and 
K  are  fitted  with  similar  springs,  for  a  similar  purpose. 
We  come  now  to  the  diaphragm  and  points.  The 
diaphragm,  which  receives  the  voice,  is  fixed  over  the 
circular  hole  in  D,  as  shown  in  Fig.  63.  It  consists  of  a 
circular  plate  of  very  thin  iron  4  in.  diameter.  Ferro- 


174  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 

type  plate  will  answer  the  purpose  very  well,  but  thin 
charcoal  iron  is  better.  It  is,  however,  possible  to  have 
the  iron  too  thin.  I  should  say  that  about  two-thirds 
the  thickness  of  an  ordinary  ferrotype  plate  is  the  best. 
The  point  is  made  from  a  knitting  needle  about  -^th 
of  an  inch  in  diameter,  which  must  be  very  hard — one 
which  can  be  bent  is  no  use.  The  original  point  had 
better  be  broken  off,  and  a  new  one  ground  upon  an 
oilstone.  For  this  purpose,  the  needle  is  held  at  an 
angle  of  about  30  degrees  with  the  stone,  and  is  con- 
stantly turned  round.  The  point,  having  been  made 
tolerably  sharp,  is  polished  and  cut  off  with  a  file.  The 
part  so  cut  off  is  T\ths  of  an  in.  long.  This  has  now 
to  be  attached  perpendicularly  to  the  centre  of  the 
diaphragm,  and  the  method  of  doing  so  is  as  follows  : — 
The  diaphragm  is  laid  upon  a  sheet  of  ^lass,  and  a 
little  spot  in  its  centre — about  \  in.  in  diameter— is 
scraped  clean  with  a  knife.  This  must  be  done  care- 
fully and  gently,  or  a  bulge  will  be  produced.  The 
fragment  of  knitting  needle  is  then  taken  up  with  pliers, 
and  its  blunt  end,  having  been  moistened  with  solder- 
ing fluid,  is  held  above  the  flame  of  a  spirit  lamp,  and 
touched  with  a  piece  of  tinfoil.  With  a  little  manipu- 
lation a  small  bead  or  globule  of  tin  may  thus  be  made 
to  adhere  to  the  end.  The  scraped  spot  on  the  dia- 
phragm is  now  moistened  with  soldering  fluid,  and  the 
diaphragm  is  supported  at  some  distance  above  a  small 
spirit  flame.  The  ring  of  a  retort  stand  forms  a  con- 
venient support.  The  butt  end  of  the  point,  with  f.in 
globule  attached,  is  then  applied  to  the  scraped  spot 


ELECTRO-MOTORS.  175 

with  pliers.  In  a  few  seconds  the  globule  melts,  when 
the  lamp  is -instantly  removed,  and  the  point  manipu- 
lated with  the  pliers,  so  as  to  be  perfectly  upright  when 
the  tin  hardens,  which  will  take  place  in  a  few  seconds 
more.  The  point  will  then  be  found  to  be  firmly  at- 
tached. The  diaphragm  and  point  must,  after  this 
operation,  be  thoroughly  washed  with  soap  and  water, 
and  slightly  oiled,  otherwise  they  will  rust.  The  only 
precaution  to  be  observed  is  to  apply  no  more  heat  than 
is  just  necessary  for  melting  the  tin.  Too  much  heat 
wiil  warp  the  disc  and,  if  it  is  a  ferrotype,  blister  the 
japan.  The  soldering  fluid  consists  of  equal  parts  of 
hydrochloric  acid  and  water,  in  which  is  dissolved  as 
much  zinc  as  possible.  A  pile  of  books  will  be  found 
useful  for  steadying  the  arm  while  manipulating  the 
point.  The  diaphragm  is  fixed  in  its  place  by  means  of 
a  brass  flange  (like  a  camera  flange),  4  in.  in  outside 
diameter,  with  a  2|  in.  opening.  Four  screws  are  used. 
The  second  diaphragm  is  made  of  parchment  paper, 
like  that  used  for  covering  jam  pots.  It  is  4  in.  in 
diameter,  and  is  gummed  over  the  hole  in  E  on  the  side 
remote  from  the  cylinder  (see  Fig.  64).  When  the  gum 
is  dry  the  diaphragm  is  moistened,  and  again  allowed 
to  dry,  when  it  will  be  found  to  be  as  tight  as  a  drum. 
The  second  point  is  exactly  like  the  first,  though  it  may, 
with  advantage,  be  a  trifle  sharper.  It  is  not  attached 
directly  to  the  paper  diaphragm,  but  to  a  steel  spring, 
which  may  be  seen  in  Figs.  63  and  64,  This  is  a  piece  of 
main  spring  ^  in,  wide  and  2|  in.  long.  It  is  fixed 
above  the  hole  in  E,  by  means  of  two  strips  of  brass,  as 


176  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 

shown  in  Fig.  64,  and  is  just  so  much,  bent  that  the  end 
of  it,  when  free,  is  f  in.  distant  from  the  plane  of  E. 
The  power  of  this  spring  may,  however,  be  varied 
within  considerable  limits  without  appreciable  diffe- 
rence in  its  performance.  The  point  is  fixed  to  the 
spring  in  the  same  manner  as  to  the  iron  disc,  but  the 
same  care  as  to  overheating  is  not  requisite,  and  the 
operation  is  consequently  easier.  Before  the  spring  is 
screwed  in  its  placed,  a  loop  of  sewing  silk  is  attached, 


FIG.  65. 

^  to  the  centre  of  paper  diaphragm  by  means  of  a  piece" 
of  court  plaister,  \  in.  square  (see  Fig.  65,  where  A  is  the 
piece  of  plaister,  B  the  loop).  The  length  of  the  loop 
must  be  such  that  when  it  is  drawn  over  the  spring  just 
above  the  point,  the  end  of  the  spring  may  be  nearly  in 
the  plane  of  E.  The  spring  is  thus  caused  to  draw  the 
paper  drum  even  tighter  than  before,  and  its  inner  sur- 
face is  rendered  slightly  convex.  Another  flange  carry- 
ing a  short  rim  or  spout  is  now  screwed  round  the  paper 
drum.  A  paper  resonator  is  made  to  slip  over  the 
short  rim  or  spout.  It  is  a  cone  made  of  two  or  three 
thicknesses  of  stout  drawing  paper.  Its  length  is  18  in.  ; 
diameter  at  small  end  2§  in.,  and  at  large  end  7  in. 
The  resonator  greatly  reinforces  the  sound  v/hen  the 
phonograph  is  speaking.  A  w.ooden  mouthpiece,  like 
those  used  for  speaking  tubes,  is  inserted  into  the  hole, 


THE  PHONOGRAPH.  177 

F  (see  Figs.  63  and  64).  The  instrument  is  now  complete, 
but  it  will  require  careful  adjustment  before  it  can  be 
used.  In  the  first  place  the  screws  which  attach  it 
to  the  base  must  be  loosened.  The  milled  nut  on  A 
screwed  up  tight,  and  the  piece  I,  shifted  about  until 
the  point  on  the  iron  disc  is  exactly  in  the  middle  of 
one  of  the  grooves  on  the  cylinder,  and  barely  touches 
the  bottom  of  it.  Then  the  screws  must  be  tightened, 
and  this  part  of  the  apparatus  finally  adjusted.  The 
same  process  is  repeated  on  the  other  side  ;  but  in  this 
case  the  adjustment  is  not  quite  final,  as  will  hereafter 
be  seen.  The  next  thing  is  to  procure  suitable  tinfoil. 
This  should  be  rather  stout — about  15  square  fe.et  to 
the  Ib. — and  should  be  cut  into  pieces  14!  in.  by  4^  in. 
Before  putting  a  tinfoil  on  the  cylinder  the  two  nuts 
are  removed,  and  the  diaphragms  turned  back  out  of 
the  way.  A  little  gum  brushed  along  one  end  of  the 
tinfoil  will  be  sufficient  to  keep  it  firmly  in  its  place  ; 
the  join  must  be  carefully  smoothed. 

"The  diaphragms  are  then  turned  back  to  their  places, 
and  the  nuts  screwed  on.  The  nut  on  G  is  screwed  up^ 
just  far  enough  to  cause  the  point  on  the  iron  diaphragm 
to  touch  the  tinfoil  very  lightly.  The  handle  is  then 
turned  about  a  quarter  of  a  revolution,  causing  the 
point  to  make  a  short  scratch  on  the  tinfoil.  The  nut 
on  G  is  thereupon  loosened,  withdrawing  the  point 
from  the  tinfoil,  and  the  nut  on  H  being  screwed  up, 
another  turn  is  given  to  the  handle.  If  the  scratch 
thus  produced  exactly  coincides  with  the  former  one, 
well  and  good ;  if  not,  the  screws  attaching  K  to  the 


178  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 

base  must  be  loosened,  and  K  shifted  about  until 
absolute  coincidence  is  attained.  The  utmost  accuracy 
on  this  point  is  essential.  The  instrument  may  now  be 
considered  fit  for  use.  Loosen  the  nut  on  H,  so  that 
the  point  on  the  spring  may  be  well  away  from  the 
tinfoil,  and  screw  up  the  nut  on  G,  so  far  that  the  point 
on  the  iron  diaphragm  may  score  a  well-defined  furrow- 
on ,  the  tinfoil  when  the  handle  is  turned.  Turn  the 
han'dle  with  as  great  regularity  as  possible,  at  the  rate 
of  about  one  turn  per  second,  or  a  little  slower.  Speak 
loudly  and  distinctly  into  the  mouthpiece,  putting  the 
mouth  as  near  as  possible  to  it,  without  actually, 
touching  it.  When  you  have  finished,  ^withdraw  the 
point  by  loosening  the  nut,  turn  £he  handle  backwards 
until  the  cylinder  is  in  its  original  position,  and  screw, 
up  the  second  nut  until  the  second  point  presses  lightly 
but  steadily  upon  the  bottom  of  the  furrow.  Then  put 
on  the  paper  resonator,  and  turn  the  handle  at  the  same 
speed  as  before.  If  the  adjustments  are  perfect,  the 
result  will  be  astonishing.  I  will  conclude  with  a  few 
general  remarks  and  hints.  It  will  be  observed  that 
this  instrument  has  two  diaphragms,  whereas  Edison's 
latest  have  only  one,  which  does  both  the  receiving  and 
the  speaking.  I  have  made  many  experiments  with 
the  object  of  dispensing  with  one  of  the  diaphragms, 
but  I  have  never,  under  any  circumstances,  obtained  so 
good  results  with  one  as  with  two.  Mr.  Preece  told  the 
Physical  Society  that  the  employment  of  only  one  was 
a  '  retrograde  step.'  Reason  and  experience  led  me  to 
concur  in  his  opinion.  I  do  not  believe  that  iron  and 


THE  PHONOGRAPH.  179 

parchment  paper  are  the  best  possible  materials  for  the 
diaphragms,  though  they  are  better  than  any  others 
which  I  have  hitherto  tried.  The  great  fault  in  the  iron 
appears  to  be  in  its  tendency  to  resound  forcibly  to 
cerlain  overtones,  in  certain  vowel  sounds.  I  have 
tried  to  overcome  this  with  coating  the  diaphragm  with 
indiarubber,  but  with  no  great  success.  I  think,  how- 
ever, that  a  ring  of  indiarubber  between  the  diaphragm 
and  the  flange  has  amundoubted  effect  in  diminishing 
the  nuisance.  The  steel  spring  is  subject  to  inde- 
pendent vibrations  of  a  similar  nature.  These  may  be 
damped  by  causing  a  piece  of  soft  indiarubber  to  press 
lightly  upon  it  at  a  point  about  \  in.  below  the  lower 
strip  of  brass.  I  have  also  found  it  an  advantage  to 
wrap  indiarubber  round  the  top  of  the  spring  before 
screwing  it  on. 

"  I  believe  that  the  mouthpiece  of  a  telephone  would 
give  better  results  than  that  of  a  speaking-tube.  A 
long  resonating  mouthpiece  like  that  which  Edison  first 
used  is  worse  than  useless.  The  point  on  the  steel 
spring  should  be  made  to  turn  very  slightly  upwards, 
instead  of  being  perpendicular.  In  the  latter  case,  it  is 
liable  to  produce  a  squeak  something  like  that  of  a 
pencil  when  drawn  up  a  slate.  If  the  points  are  too 
sharp,  they  will  cut  and  scrape  the  tinfoil  ;  if  too  blunt, 
the  articulation  will  be  muffled.  After  the  points  have 
traversed  the  cylinder  200  or  30x3  times,  they  will 
require  sharpening.  This  can  be  done  with  a  small 
oil-stone,  without  removing  them." 


I8o  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 

$  104.    THE  MICROPHONE. 

A  simple  and  yet  sensitive  form  of  this  instrument 
may  be  made  as  follows : — 

Procure  a  piece  of  good  graphite  from  the  gasworks, 
and  cut  from  it  with  an  old  saw  three  rods  about  f  in- 
square  by  2  in."  long.  If  these  are  roughly  cut  out 
with  the  saw,  they^can  be  finished  up  beautifully  by 
rubbing  on  a  flat  stone,  with  a  little  fine  sand  and 
water.  When  this  has  Been  done,  they  should  be  put 
in  an  oven  to  dry.  A  piece  of  planed  deal  8  in.  by 
3i  in.  should  now  be  procured,  and  a  length  of  3  in. 
cut  off  one  end,  and  glued  and  screwed  to  the  longer 
piece  so  as  to  form  a  letter  L.  A  line  is  now  drawn 
down  the  centre  of  the  longer  piece ;  along  this  line,  at 
equal  distances  from  the  two  ends  of  the  piece,  and 
about  i|  in.  from  each  other,  two  square  holes  are 
cut  into  the  board,  of  such  a  size  as  to  admit 
ihe  square  ends  of  two  out  of  the  three  graphite 
rods  just  prepared.  Previous  to  inserting  the  carbon- 
rods  in  these  holes  permanently,  these  holes  must  be 
bushed  with  thin  sheet  copper  (the  same  as  is  used  for 
dynamo  brushes).  This  bushing  serves  to  make  a 
good  connection  between  the  carbon-rods,  and  two 
binding  screws  inserted  into  the  shorter  board  which 
forms  the  base  of  the  instrument.  This  is  effected  by 
soldering  a  piece  of  No.  20  copper  wire  to  each  bushing, 
and  bring  a  wire  from  each  to  its  respective  binding- 
screw.  •  For  the  sake  of  neatness,  the  wires  may  be 
taken,  out  at  the  back  of  the  L,  and  brought  round  under- 


THE  MICROPHONE.  '181 

neath  to  the  two  binding  screws,  a  channel  being  cut 
with  a  penknife  to  allow  the  wires  to  He  in.  The  third 
piece  of  graphite-rod,  which  has  not  hitherto  been 
brought  into  use,  is  now  sharpened  at  both  extremities 
(by  rubbing  on  a  flat  stone  with  sand),  so  as  to  produce 
conical  pyramids,  with  sharp  points  at  both  ends  of  the 
rod.  By" means  of  a  \  in.  bit,  put  in  the  Archimedean 
drill,  a  slight  depression  is  made  at  about  J  in.  from  the 
edge  of  each  of  the  carbons  which  have  been  fitted  to 
the  holes  in  the  upright  backboard  of  the  L.  These 
depressions  must  be  in  the  inside  (on  the  upper  surface 
of  the  lower  carbon,  and  on  the  lower  surface  of  the 
upper  carboti),  and  serve  to  support  the  pyramidical 
graphite  by  its  two  opposite  points.  To  adjust  this 
properly,  the  upper  carbon  is  removed  bodily ;  the 
lower  carbon  pushed  into  the  lower  bushed  hole  until 
it  is  flush  with  the  back  of  the  board.  If  it  is  not  quite 
firm  in  this  position,  small  pieces  of  copper  may  be 
wedged  in,  until  it  is  quite  firm.  The  double-pointed 
carbon-rod  is  now  placed  with  its  lower  point  resting  in 
the  depression  in  the  lower  rod,  and  is  held  thus  while 
the  upper  carbon  is  being  inserted  into  the  upper 
bushed  hole,  care  being  taken  that  the  upper  pointed 
end  of  the  loose  carbon-rod  is  resting  in  the  depression, 
on  the  under  surface  of  the  upper  carbon-rod.  The 
upper  carbon  is  now  pushed  into  the  upper  hole  until 
flush  with  the  back  of  board.  Care  must  be  taken  that 
there  must  be  sufficient  space  between  the  upper  and 
lower  depressions  to  allow  the  central  carbon-rod  tc 
turn  quite  freely  on  its  axis,  but  not  so  much  as  to  le| 


X82  ELECTRICAL  INSTRUMENT  MAKING  FOR  AMATEURS. 

it  fall  out.  This  microphone,  if  glued  by  its  shortest 
base  to  the  bottom  of  an  empty  cigar-box  (turned 
upside  down),  from  which  the  lid  has  been  removed, 
will  be  found  extremely  sensitive.  For  use,  connect 
one  binding-screw  to  one.  pole  of  a  small  Leclanche. 
Connect  the  other  binding  screw,  by  means  of  a  long 
wire,  to  a  telephone,  and  bring  a  return  wire  from  the 
telephone  to  the  other  pole  of  battery.  The  tick  of 
a  watch,  the  sound  made  by  the  walking  of  a  fly  can  be 
distinctly  heard  at  the  telephone  end,  if  the  fly  or  the 
watch  is  placed  on  the  cigar-box  forming  the  sounding- 
board  of  the  instrument. 

WATER  REGULATOR  TOR  INDUCTION  COILS. — 
Instead  of  the  sliding  tube  arrangement  described 
at  §  49,  it  is  sometimes  convenient  to  have  a 
separate  regulator  of  the  strength  of  shocks,  etc.  The 
following  will  be  found  very  easy  to  make.  Procure  a 
stout  glass  tube  i  in.  bore,.  1 2  in.  long.  Fit  it  with  a  sound 


FIG.  66. 

cork  at  each  end,  run  a  short  length  of  No.  16  copper- 
wire  through  the  centre  of  one  cork,  and  a  longt  length 
(about  II  in.)  through  the  other.  Make  a  ring  of 
the  wire  at  each  outer  extremity.  Put  one  cork  in,  fill 
the  tube  with  water,  then  insert  the  other  cork.  Wipe 
dry,  then  sealing-wax  the  outsides  of  corks.  The  more 
the  long  wire  is  drawn  out,  the  weaker  will  be  the  shock 
from  the  coil,  when  this  regulator  forms  part  of  the 
circuit. 


APPENDIX    TO    TABLE    OF    WIRE 
RESISTANCES,  ETC.,  §  73. 

It  is  frequently  necessary  to  know  what  diameter  of  wire 
must  be  used  to  carry  a  current  safely ;  id  esf,  without 
injurious  heating.  The  following  table  gives  approximately 
the  ratio  between  the  gauge  of  the  wire,  and  the  safe  current 
in  amperes,  which  the  wire  can  carry. 


B  W  G 
6 


9 
10 
ii 

12 

13 
14 

!i 

17 
18 
19 

20 
22 


SAFE  CURRENT  IN  AMPERES. 
60 


43 

8 

21 

IS- 

13 

IO 


5 

3 

2'5 

2 


The  above  table  refers  to  ordinary  commercial  copper 
wire  only ;  it  is  NOT  applicable  to  any  wire  of  different 
material,  such  as  brass,  lead,  tin,  platinum,  German 
silver,  &c.  Even  pure  copper  differs  considerably  from 
the  ordinary  commercial  article,  in  carrying  power;  somfe 
samples  tried  by  me  having  a  "  safe  carrying  capacity  " 
of  nearly  double  that  given  in  the  table. 


APPENDIX  TO  GALVANOMETERS,  §  86. 

PREVIOUS  to  publication  in  a  complete  form,  the  fore- 
going chapters  on  *'  Electrical  Instrument-making  for 
Amateurs"  have  elicited  a  considerable  amount  of  corre- 
spondence from  the  readers  thereof  and  many  kind  sugges- 
tions from  friends  of  the  author.-  Foremost  among  these,  and 
replete  with  valuable  hints  as  to  the  desirability  of  encourag- 
ing pupils  and  teachers  to  make  their  own  instruments,  is 
one  by  Mr.  Wm.  Robertson,  of  Castle  Douglas,  N.B.  Him- 
self a  teacher,  he  has  had  practical  experience  of  the 
thoroughness  of  the  knowledge  acquired  by  students  who 
learn  to  make  as  well  as  to  use  their  own  instruments.  In 
these  remarks  the  author  fully  concurs.  During  the  ten 
years  in  which  he  was  engaged  in  teaching  the  science  of 
electricity  at  the  Carshalton  House  School,  he  invariably 
adopted  the  plan  of  causing  the  pupils  to  construct  the 
apparatus  which  formed  the  substance  of  each  lesson  ;  and 
the  result  has  been  that,  although  the  apparatus  turned  out 
may  have  been  of  the  roughest  description,  yet  the  insight 
obtained  by  the  students  has  been  so  thorough  as  to  enable 
them  to  pass  examinations,  etc.,  much  more  creditably  than 
in  cases  where  book  knowledge  alone  was  imparted. 

Feeling  that  the  subject  may  interest  others,  he  ventures 
to  reproduce  here  a  portion  of  Mr.  Robertson's  letter,  referring 


APPENDIX.  tt$ 

more  especially  to  the  construction  of  two  novel  forms  of 
galvanoscspe,  eminently  adapted  to  show  the  laws  which 
regulate  the  action  of  currents  on  magnets,  etc. 

"  Permit  me,"  says  Mr.  Robertson,  "  to  make  a  suggestion. 
I  have  found  that  in  teaching  no  apparatus  is  half  so  valuabU 
as  that  which  is  home-made.  I  have  constructed  several 
simple  galvanometers  (or  rather  galvanoscopes)  which  have 
proved  very  useful.  Even  if  the  teacher  himself  has  learned 
to  make  a  '  fine  job  '  he  will  find  it  of  advantage  to  use  very 
plain  and  simple  apparatus  in  his  class  work.  In  this  way 
the  students  are  encouraged  to  try  their  own  hands  at  ap» 


Fio.  58. 

paratus  making.  Occasionally  my  pupils  have  made  fairly 
good  '  galvanoscopes '  at  home,  even  in  spite  of  their 
mammas,  and  their  dislike  to  a  mess  at  the  fireside. 

"  Inclosed  you  will  find  sketches  of  two  novel  forms  of 
'galvanoscopes,'  by  means  of  which  action  of  currents  on 
magnets  can  be  easily  and  accurately  studied." 

The  first  instrument  is  figured  at  Fig.  58.  It  consists  of  a 
base  board,  about  8  in.  by  5  in.  by  £  in.,  shown  at  b.  In  the 
middle  pf  this  is  placed  a  stout  brass  wire,  bent  into  the 
form  of  a  ring,  with  the  ends  prolonged  at  e}  e'}  where  they 


|g£  APPENDIX. 

are  held  down  to  tho  base  board  by  tne  two  binding  screws, 
*,  y.  At  the  point  where  the  r/ire  crosses  itself  to  form  the 
hoop  (which  should  be  about  4  in.  in  diameter),  the  wire  is 
wrapped  round,  on  both  portions,  with  a  layer  or  two  of  silk 
ribbon,  which  serves  at  once  to  separate  them  from  electrical 
contact  witV  each  other,  and  to  retain  the  wire  in  the  shape 
of  a  ring.  At  the  upper  portion  of  the  ring  is  fastened  a 
little  hook,  £,  which  is  held  in  position  by  being  neatly 
bound  round  with  a  little  silk  twist. 

At  each  extremity  of  the  stand  is  a  wide  strip  of  sheet 
brass,  bent  at  right  angles  x  *',  of  the  shape  figured.  Each 
strip  is  secured  to  the  base-board  by  two  small  screws,  c  c', 
and  furnished  with  a  binding-screw,  K.  K',  to  make  connection 
with  a  battery  if  required.  Each  of  these  strips  (the  centre 
of  which  should  coincide  with  the  central  plane  of  the  ring) 
is  perforated  with  four  equidistant  holes,  one  on  each-  side 
of  the  centre  and  one  above  and  below.  These  holes  are 
to  permit  the  introduction  of  a  straight  wire  or  wires,  above, 
below,  or  on  either  side  of  the  centre  of  the  ring.  These 
wires  serve  to  conduct  the  current  from  one  strip  to  the 
Dther.  Two  magnetic  needles  may  be  used  with  this  in- 
strument— namely,  one  free  to  move  vertically  only,  as 
figured  at  M ;  and  another  so  suspended  as  to  swing 
horizontally.  The  former  is  furnished  with  two  hooks,  by 
which  it  may  be  temporarily  held  by  the  ring  ;  the  latter  is 
provided  with  a  fine  silk  suspension,  which  can  be  slung  on 
to  the  hook  h  in  the  ring.  This  instrument,  which  Mr. 
Robertson  calls  the  "  Deflector,"  may  be  used  in  the  following 
mode: 

I.  Having  placed  the  "deflector"  with  the  brass  strips 
pointing  north  and  couth,  hang  the  "vertical-"  needle  M 
from  the  top  cf  the  ring.  Now  insert  a  brass  or  copyer  wire 


APPENDIX.  187 

through  the  upper  holes  in  the  strips,  and  couple  up  the 
binding-screws  on  the  strips  with  a  battery.  No  Deflection 
ensues. 

2.  Place  the  wire  in  the  lower  pair  of  holes,  and  again 
Couple  up.     Still  no  deflection. 

3.  Now  place  the  connecting  wire  in  the  right-hand  pair 
Of  holes.     Deflection  takes  place. 

4.  Place  the  connecting-wire  in  the  left-hand  pair  of  holes. 
Deflection  ensues  in  the  opposite  sense. 

5.  Remove   the    "  vertical "     needle,    and   substitute   t'ie 
"  horizontal."     Now  place  the  wire  connector  in  the  pair  of 
holes  ovef  the  needle  ;  couple  to  battery.     Deflection  ensues 
in  one  direction. 

6.  Place  the  •wire  connector  in  the  pair  r,f  holes  under  the 
needle.     Deflection  is  obtained  in  the  opposite  direction. 

7.  Place  two  exactly  similar  wires,  one  in  the  pair  of  holes 
above  and  the  other  in  the  pair  of  holes  bslow  the  "  horizontal " 
needle.     On  coupling  up   to  battery  no  deflection  occurs,  if 
the  two  wires  are  equidistant  from  needle,  and  if  both  be  of 
equal  conductivity.     This  is,  however,  seldom  the  case  ;  and 
then   the   nearer,    or    better    conducting     wire,    rules    the 
deflection. 

8.  Wires  of  different  gauge,  but  of  same  material. 

9.  Wires  of  same  gauge,  but    of  different   material.     In 
these  two  latter  cases  the  needle  obeys  the  better  conductor, 
provided  the  distances  be  equal. 

10.  Change   the  position    of  the   instrument,    place  the 
plane  of  the  ring  in  the  magnetic  meridian,  remove  all  the 
wires  out  of  tho  hole  in  the  strips,  suspend  the  horizontal 
needle  in  the  ring,  and  when  it  is  at  rest  in  the  plane  of  the 
ring,   pass  current  through  the  binding  screws  connectod 
v/ith  the  ring  s  /.    Defoctivn  ensues* 


l88  APPENDIX. 

ii.  Reverse  the  direction  of  the  current.  Deflection  in 
Apposite  direction. 

The  other  instrument,  or  "differential  galvanoscope," 
consists  simply  in  a  base  board,  as  shown  at  Fig.  59.  B,  to 
which  is  fastened  by  four  binding  screws,  A  A  and  c  c',  a 
ring  R  made  of  two  insulated  wires  lying  side  by  side,  ths 
extremities  of  which  are  connected  to  the  four  binding  screws. 
At  right  angles  to  the  plane  of  this  ring  rises  a  light  rectan- 
gular frame  of  stout  brass  wire  F,  on  the  top  of  which  slides 
with  friction  a  small  spiral  of  wire,  terminating  in  a  hook 
Q.  (This  serves  to  suspend  the  magnetic  needle.) 


A  short  piece  of  brass  wire,  bent  twice  at  right  angles,  \V, 
called  the  "bridge,"  serves  to  connect  the  nearer  pair  of 
binding-screws,  c  c',  when  required.  A  "  horizontal "  mag- 
netised needle,  suspended  by  silk,  may  be  hung  in  the  middle 
of  the  ring,  or  to  one  or  other  side  of  it,  by  means  of  the 
hook  Q. 

This  instrument  can  be  used  to  prove  the  counteracting 
effect  of  currents  in  opposite  directions  ;  since,  if  the  bridge 
W  be  inserted  in  the-  binding-screws  c  c',  and  a  current  be 
sent  round  by  the  binding-screws  A  A',  it  traverses  the  two 


APPENDIX.  183 

rings  in  opposite  directions.  Hence  the  needle  is  not  deflected 
if  perfectly  central ;  but  if  the  needle  be  to  one  side,  the 
•nearer  circle  governs  the  needle.  Having  adjusted  the 
needle  to  perfect  centrality  by  sliding  the  spiral  Q  until  the 
needle  is  unaffected  by  any  current  passing,  the  instrument 
rtlay  be  used  for  comparing  the  current  given  by  different 
cells.  To  this  end  it  is  only  necessary  to  remove  the  bridge 
arid  couple  up  one  cell  to  A  c  and  the  other  to  A'  c\  care 
being  taken,  of  course,  to  send  the  currents  in  fr-he  same 
direction. 


APPENDIX  TO  BATTERIES  §  95. 

/ACCUMULATORS,  OR  STORAGE  BATTERIES. — If  a  current 
of  electricity  traverse  an  electrolyte  (any  compound  body 
capable  of  allowing  the  passage  of  electricity)  decomposition 
of  that  body  takes  places.  The  surface  of  the  conductor 
by  which  the  current  enters  (the  positive  electrode  or  anode) 
attracts  to  itself  the  more  negatii^e  constituents  of  the  said 
body  ;  while  the  conductor  by  which  the  current  leaves  the 
decomposition  cell  (the  negative  electrode  or  kathode)  in  like 
manner  attracts  to  itself  the  more  positive  constituents  of 
the  compound.  As  this  decomposition  is  only  effected  by 
means  of  the  electric  strain  put  upon  the  constituents  of  the 
compound,  it  is  evident  that  if  the  strain  (the  original 
electric  current)  cease,  the  effect  will  also  cease,  and  recorn- 

NOTE. — For  a  more  exhaustive  work  on  this  subject,  the  rear!**  « 
referred  to  Sir  D.  Salomon's  practical  handbook,  "  Maiucemeni  of 
Accumulators  and  Private  Electric  Light  Installation  "  (Whitiiker  a*>d 
Co.),.  1 5 88. 


190  APPENDIX. 

position  will  be  effected  ;  in  other  words,  the  original  elec- 
trolyte will  be  reformed,  and  a  current  of  electricity  set  up 
in  the  opposite  direction.  (See  §  81).  We  may  liken  the 
effect  of  the  current  to  a  force  exerted  against  a  spring,  and 
the  electrolyte  to  the  spring.  When  the  force  exerted  is 
removed,  the  spring,  in  recovering  itself  from  the  strain, 
exerts  a  force  in  the  opposite  direction  to  that  originally 
employed. 

Based  on  a  knowledge  of  these  facts,  and  an  application  of 
Ohm's  law  (§  82),  we  are  able  to  construct  an  apparatus 
which  shall  return  to  us  as  electrical  energy  a  considerable1 
percentage  of.  the  current  passed  through  it,  and  at  will  to 
cause  the  energy  to  manifest  itself  either  as  a  large  current 
at  a  low  pressure,  by  diminishing  the  internal  resistance  ; 
or  as  a  smaller  current  at  a  high  pressure,  by  arranging  the 
elements  in  series,  so  as  to  get  cumulative  effects. 

The  first  accumulator  ever  made  was,  perhaps,  that  of 
Ritter,  who  in  1803  constructed  a  secondary  pile  of  a  number 
of  discs  of  similar  metal,  separated  by  pieces  of  moistened 
cloth.  On  connecting  for  a  few  seconds  the  opposite  extre- 
mities of  this  pile  with  the  poles  of  a  battery,  the  pile  will 
be  found  to  have  acquired  the  power  of  producing,  for  a  shor 
time,  a  current  opposite  to  that  of  the  battery. 

The  next  to  utilize  the  polarization  current  was  Grove, 
who  devised  what  is  known  as  the  "  gas  battery."  This 
consists  of  two  platinum  plates,  standing  upright  in  a  vessel 
of  water,  each  plate  being  surrounded  by  a  glass  tube  closed 
at  the  top  with  a  platinum  wire  in  connection  with  each 
plate  sealed  in  the  top  of  each  tube,  and  projecting  out  of  it. 
On  passing  a  current  through  the  water  by  means  of  the 
projecting  wires,  the  water  is  decomposed,  the  oxygen  col- 
lecting in  the  tube  at  which  the  current  enters,  and  the 


APPENDIX. 


191 


hydrogen  in  that  at  which  it  leaves.  On  Interrupting  the 
main  current,  and  connecting  up  the  two  platinum  wires,  a 
current  is  set  up  in  the  opposite  direction,  while  the  hydrogen 
and  oxygen  recombine  to  form  water. 

To  Gaston  Plant£  is,  however,  due  the  honour  of  having 
made  the  accumulator  a  practically  useful  instrument. 

THE  PLANT^  ACCUMULATOR,  which  for  efficiency  has  not 
yet  been  surpassed,  may  be  made  by  taking  two  sheets  of 
r\-  in.  lead,  each  about  6  in.  wjide  by  3  ft.  long,  placing  one 
on  a  flat  table,  then  placing  lengthwise  on  this  sheet  three 
strips  of  india-rubber,  a  trifle  longer  than  the  leaden  sheet, 
but  only  \  in.  wide,  at  equal  distances  from  each  other 
Over  these  is  to  be  laid  the  second  leaden  sheet,  and  over 
this  latter  again,  three  india-rubber  strips,  similar  to  the  first 
three.  (These  serve  only  to  keep  the  sheets  from  contact 
with  each  other,  and  may  be  replaced  by  asbestos  cloth  or 
any  insulator  not  acted  on  by  acids).  The  two  sheets  are 
then  to  be  rolled  into  a  tight  spiral  on  a  wooden  cylinder. 
A  leaden  strip  or  lug  is  soldered  to  one  end  of  each  sheet  to 
serve  as  terminals,  for  connection. 

A  cylindrical  glass  or  earthenware  vessel  (glazed),  of 
sufficient  size  to  contain  the  spiral,  but  leaving  the  lugs  pro- 
jecting above  the  upper  extremity,  is  now  fitted  with  a 
paraffined  wood,  or  ebonite  cover,  having  two  apertures  to 
admit  of  the  passage  of  the  lugs.  The  spiral  having  been 
tied  together  on  the  outside  with  a  gutta-percha  or  india- 
rubber  band,  may  now  be  inserted  in  the  containing  vessel, 
into  which  must  have  previously  been  poured  the  follow- 
ing  mixture,  to  reach  nearly  to  the  top  of  the  spiral,  when 
this  latter  is  inserted  into  the  jar,  viz.  : 
Water  ...  ic  parts  1 
Sulphuric  Acid  i  part  J  b?  measur*« 


I9i  APPENDIX. 

N.B. — In  mixing  sulphuric  acid  with  water,  it  ^nust  he 
borne  in  mind  that  the  acid  must  be  added  in  a  fine  stream 
to  the  water,  stirring  with  a  glass  rod  in  the  meantime,  and 
not  the  water  to  the  acid  ;  otherwise  the  violent  reaction 
may  cause  a  dangerous  accident.  .  The  Plante  cell  thus 
constructed  has  to  be  "  formed,"  that  is  to  say  a  current  of 
electricity  has  to  be  passed  into  it  until  small  bubbles  of 
oxygen  gas  show  themselves  at  the  anode,  then  discharged, 
then  again  charged  in  the  opposite  direction,  and  again  dis- 
charged, and  so  on  for  a  fortnight  or  more,  until  the  surfaces 
of  the  leaden  sheets,  by  continual  oxidation  and  deoxidation 
have  become  sufficiently  spongy  to  retain  a  considerable 
charge.  It  is  this  "  forming "  that  constitutes  the  great, 
and,  indeed,  the  only  objection  to  the  Plante  accumulator. 
The  time  necessary  to  effect  the  "  formation "  may  be 
greatly  shortened,  as  shown  by  Plante  in  1883,  by  immers- 
ing the  leaden  sheets  in  a  20  per  cent,  solution,  of  nitric 
acid,  previous  to  subjecting  them  to  the  action  of  he  charg- 
ing current.  Faure,  later  on,  showed  that  the  long  and 
tedious  process  of  "  forming  "  the  plates  by  the  influence  of 
the  current,  might  be  still  farther  abbreviated,  by  mechani^ 
cally  dressing  the  surfaces  of  the  plates  with  lead  in  an 
oxidised  condition.  This  is  effected  by  making  a  paste  of 
red  lead  and  sulphuric  acid,  and  smearing  the  surfaces  with 
this  paste,  previous  to  rolling  up  the  sheets.  This  enables 
the  accumulator  to  be  used,  after  it  has  been  charged  two 
or  three  times  only.  Innumerable  "patents"  and  improve- 
ments have  been  made  on  this  device,  such  as  punching, 
honeycombing,  or  gridding  the  leaden  plates,  to  enable 
them  to  retain  the  paste,  which  is  very  apt  to  fall  away 
from  the  surface  during  action. 


APPENDIX.  193 

The  following  data  as  to  the  E.M.F.  and  capacity  of 
accumulators  may  be  of  interest : — 

ist.  Each  cell,  irrespective  of  size  and  number  of  plates  it 
may  contain,  provided  these  are  connected  to  form  virtu- 
ally but  TWO  plates,  will  have  an  E.M.F.  of  about  2-25  volts 
when  first  charged,  quickly  falling  to  about  2  volts,  at  which 
it  remains  steady  until  nearly  exhausted.  When  the 
E.M.F.  falls  to  i'9  volt,  it  is  a  sign  that  the  discharge  should 
be  stopped,  as  it  is  not  advisable  to  completely  discharge  the 
cell. 

2nd.  Every  square  foot  of  surface  of  the  positive  plate  in  a 
well-made  accumulator  (say  the  E.P.S.  type)  is  capable  of  dis- 
charging at  the  rate  of  six  amperes  per  hc-ur,  or  one  ampere 
for  six  hours  (six  ampere  hours).  This  is  only  true  if  the 
plates  are  arranged  in  parallel,  if  they  are  arranged  in 
series,  so  as  to  obtain  a  higher  E.M.F.,  then  the  quantity  to 
be  discharged  is  equal  to  the  surface  of  the  one  element  only 
at  the  same  rate. 

3rd.  The  charging  current,  in  amperes  should  be  some- 
what less  (say  £  less)  than  that  of  discharge. 

As  a  practical  illustration  cf  the  application  of  thd  two 
first  rules :  suppose  v/e  desire  to  light  a  5  c.p.  lamp,  of  8 
hours'  resistance,  for  one  hour.  Such  a  lamp  will  require 
about  I  ampere  of  current  to  flow  through  it  to  light  it 
properly. 

Hence   to  drive   I   ampere    through   8    ohms    we  shall 

•p- 

need  an  E.M.F.  of  8  volts,  since  ~  =  C.     Therefore  4  cells 
R 

will  be  needed,  coupled  in  series.  As  I  square  foot  of  posi- 
tive surface  can  furnish  6  amperes  per  hour,  ^  of  a  square 
toot  will  be  sufficient  to  furnish  the  I  ampere  needed  foi  I 
hour.  Hence  each  csll  (as  these  are  coupled  in  series)  must 


194  APPENDIX., 

present  at  least  this  amount  of  positive  surface.  Therefore 
6  '"cells,  coupled  in  series,  each  cell  containing  a  pair  of 
plates  i  foot  long  by  2  inches  wide,  would  just  do  the  work 
desired. 

POCKET  ACCUMULATOR. — Many  amateurs  are  desirous  of 
constructing  a  small  pocket  battery,  capable  of  lighting  the 
little  4  volt  "  fairy  "  lamps,  used  for  scarf  pins  or  head 
decorations.  The  following  will  be  found  easy  to  make,  and 
effective  in  action : — Having  procured  some  good  sheet  gutta- 
percha,  \  in.  in  thickness,  let  three  squares  be  cut  from  it 
3$-  in.  x  5"  in.  each,  two  others  5  in.  x  i£  in.,  and  one 
3f  in.  X  i£  in.  This  latter  having  been  laid  on  a  smooth 
elate  (previously  moistened  on  the  surface  to  prevent  adhe- 
sion) a  rather  hot  poker  is  passed  quickly  once  or  twice  over 
its  surface,  so  as  to  render  it  soft  and  sticky.  The  narrowest 
edges  (\\  in.)  of  the  three  larger  squares  (those  5  in.  X  3^-  in,) 
are  then  lightly  rubbed  over  with  the  hot  poker,  and  imme- 
diately pressed  down  upon  the  strip  on  the  slate,  so  as  to 
divide  it  into  two  equal  spaces,  or  little  over  \  in.  wide,  by 
3^  in.  long.  Special  care  must  be  taken  that  the  three  up- 
right squares  which)  are  to  form  the  sides  and  central  parti- 
tion of  the  accumulator  box)  adhere  firmly  and  at  all  parts 
to  the  bottom  strip.  An  assistant  should  hold  these  square 
upright  and  parallel,  while  the  operator  proceeds  to  heat 
with  the  poker  the  two  remaining  side  strips  (5  in.. by  ij  in.) 
which  he  applies  while  hot  to  the  sides  o  the  cell.  The 
same  care  must  be  taken  that  perfect  adherence  takes  place 
with  these  strips  and  the  three  squares,  as  in  the  case  above 
with  these  latter  and  the  bottom.  If  this  be  not  attended 
to,  either  the  whole  cell  will  leak  into  the  carrier's  pocket 
or  the  internal  divison  will  allow  the  fluid  to  pass  from  one 
side  to  the  other.  This  latter  defect  would  be  fatal  to  the 


APPENDIX. 


'9.S 


due  action  of  the  accumulator^  To  test  this,  when  the  cell 
is  quite  cold  and  hard,  it  will  be  well  to  fill  one  side  care- 
fully with  water,  and  notice  whether  there  be  any  leakage, 
cither  through  the  sides  of  the  central- partition,  or  to  the 
outside.  Should  there  be  any  leakage  externally,  the  water 
should  be  emptied  out,  the  cell  dried,  and  the  defective  joint 
rubbed  over  externall)  with  the  hot  poker,  until  the  joint  is 
made  sound.  Should  the  defect  be  internal,  a  piece  of  sfouf 
wire  bent  into  the  shape  of  L  should  be  heated  at  one  limb, 
and  this  rubbed  over  the  defective  portion,  so  as  to  secure  a 
perfect  water-tight  compartment. 

Four  plates,  4^  in.  x  3  in.,  with  an  ear  or  lug  about  one 
in.  long  by  one  in.  wide,  must  now  be  cut  out  of  some  -fa 
in.  sheet  lead.  With  a  small  punch  or  bradawl,  these 
plates  must  be  perforated  all  over,  as  thickly  as  possible 
to  within  about  ^  in.  of  the  top,  or  lug  end.  A  thick  paste 
must  now  be  made  by  mixing  some  good  red  lead  with 
equal  parts  of  oil  of  vitriol  and  water.  The  holes  which 
have  been  punched  in  the  four  plates,  must  now  be  filled 
in  with  this  thick  paste,  by  means  of  a  flat  wooden  stick 
or  spatula.  The  surface  also  of  the  plates  must  be  liberally 
smeared  over  with  the  paste,  the  lugs  only  excepted.  The 
plates  should  now  beset  aside  for  a  short  time,  to  allow 
the  paste  to  harden.  While  this  is  taking  place,  a  piece  of 
cigar  box  should  be  qut  of-  the  exact  size  to  fit  the  top  of 
the  gutta-nercha  box  previously  made,  viz.,  3|in.  X  if  in. 
A  central  line  having  been  drawn  lengthwise  down  this, 
two  slots  ^  in.  wide  by  I  in.  long  are  cut  on  each  side  of 
the  said  line.  The  use  of  these  slots  is  to  admit  the  pas- 
sage of  the  lugs  or  ears  of  the  leaden  plates.  They  should 
be  all  parallel,  and  stand  about  J  in.  apart.  Two  cir- 
cular holes,  nearly  ^in.  in  diameter  must  also  be  drilled 


1 96  APPENDIX. 

through  this  cover,  one  on  each  side  of  the  central  line,  and 
equidistant  from  the  slots!  These  holes  serve  to  take  short 
lengths  of  glass  tube,  similar'  to  that  used  in  feeding  bottles. 
The  wooden  cover  must  now  be  soaked  in  hot  melted  paraffin 
until  thoroughly  permeated  with  it.  The  lugs  of  the  leaden 
plates  are  now  passed  through  the  slots'(which  they  ought 
to  fit  tightly),  the  two  central  ones  bent  towards  each  other 
till  they  meet,  cut  off  at  the  point  thy  meet,  and  soldered 
together.  To  each  of  the  two  outer  lugs  is  spidered  a  small 
binding  screw,  to  serve  as  terminals.  Any  excess  of  lead 
on  the  lugs  may  now  be  cut  off.  The  gutta-percha  cells 
should  now  be  nearly  three  parts  filled  with  dilute  sulphuric 
acid  (r  part  acid  to  4  parts  water)  and  the  plates  im- 
mersed therein  The  acid  should  not  reach  the  top  of  the 
cells  when  the  plates  are  in,  the  lid  or  cover  should  rest  on  the 
top  of  the  cells  all  round.  Should  this  not  be  the  case,  the 
leaden  plates  must  be  cut  a  trifle  shorter.  Two  short  lengths 
of  tube  are  now  inserted  into  the  holes  left  for  that  purpose, 
and  cemented  into  position  with  a  cement  made  of  one  part 
melted  pilch  and  two  of  gutta-percha,  applied  hot.  With  the 
same  cement,  the  top  or  cover  must  be  cemented  down  to 
the  outer  cell  ;  if  the  cover  be  cemented  all  over  (except 
the  mouth  of  the  tubes),  it  will  prevent  any  chance  leakage. 
A  little  soft  india-rubber  stopper  should  now  be  made  to 
fit  the  orifice  of  each  tube,  removable  at  will.-' 

To  charge  this  accumulator  it  should  be  connected  to 
a  small  dynamo,  or  to  a  four-cell  bichromate  or  chromic  acid 
battery  (§  98)  for  three  hours,  then  the  terminals  connected, 
so  as  to  allow  it  to  discharge  itself,  then  charged  in  the 
reverse  direction,  and  so  on  for  several  days  in  succession,  or 
until  it  is  found  that  the  accumulator  will  ring  an  electric 
bell  for  fifteen  minutes,  after  being  charged  only  ten  minutes. 


APPENDIX.  197 

When  this  occurs,  the  cell  must  be  charged  in  one  direction 
only,  the  terminals  being  marked  in  order  to  know  where  to 
connect  for  the  next  time  of  charging.  When  complete,  this 
accumulator  will  light  a  3  or  4  volt  lamp  well  for  about 
two  consecutive  hours. 

It  is  advisable  to  charge  the  accumulator  within  about  an 
hour  or  so  of  being  wanted  for  use,  so  as  to  incur  as  little 
loss  of  power  as  possible,  due  to  the  unavoidable  short- 
circuiting  which  takes  place  in  the  inside,  through  the 
damp  lid.  Previous  to  permanently  sealing  up  the  cell,  it 
will  be  necessary  to  place  a  small  piece  of  india-rubber 
between  each  pair  of  plates,  to  prevent  any  accidental 
contact  or  "  short  circuiting." 


APPENDIX  TO   DYNAMO,   §   74: 

ELECTRO-MAGNETS. — Many  amateurs  are  puzzled  how  to 
wind  tho  iron  cores  of  dynamos,  electric  bells,  &c.,  in  order 
to  obtain  desired  results. 

The  first  point  to  be  noted  is  the  amount  of  wire  to  be 
got  on  the  core.  Since  the  magnetizing  effect  of  the  current 
decreases  as  the  distance  from  the  iron  core  increases,  in 
proportion  to  the  square  of  the  distance,  it  follows  that  we 
do  not  gain  power  if  we  add  on  layers  of  coils  beyond 
a  certain  point.  In  practice,  it  will  be  found  that  no 
advantage  is  gained  in  putting  on  more  wire  than  will 
increase  the  section  of  the  completed  electro-magnet  to 
three  times  that  of  the  bare  iron  core.  Let  us  suppose  we 
had  a  core  one  inch  in  diameter  :  the  diameter  of  the 
wound  electro-magnet,  including  the  wire  coils,  should  not 
exceed  i  hue  inches, 


I9»  APPENDIX. 

The  second  point  is,  What  gauge  of  wire  must  be  used 
wherewith  to  coil  the  core  ? 

By  experiment  it  has  been  ascertained  that  the  magne- 
tizing effect  is  the  same,  whether  1,000  amperes  are  sent 
once  round  an  iron  core,  or  whether  one  ampere  is  sent 
1,000  times  round  the  same  core  ;  or,  in  other  words,  "  the 
magnetising  effect  is  proportional  to  the  number  of  ampere- 
turns  of  coil."  If,  therefore,  we  have  to  wind  an  electro- 
magnet to  be  used  with  a  large  current  of  low  E.M.F.  we 
must  wind  our  electro-magnet  with  few  coils  of  thick  wire, 
both  for  the  sake  of  carrying  this  large  current,  and  of 
avoiding  resistance.  If,  however,  the  current  at  our  dis- 
position is  small,  but  delivered  by  a  high  E.M.F.  then  we 
select  a  finer  wire. 

The  third  point  is  the  direction  of  winding. 

To  produce  a  magnet  with  normal  poles  (one  north  and 
the  other  south),  the  direction  of  winding  must  always  be 
the  same.  Let  us  suppose  we  start  winding  in  the  direction 
of  the  motion  of  the  hands  of  a  clock :  then,  although  we 
are  at  liberty  to  coil  over  the  first  layer,  any  number  of 
times,  yet  we  must  always  wind  in  that  same  direction,  viz., 
from  left  over  to  right.  This  is  true,  whatever  be  the  form 
of  the  magnet  core.  It  is  easily  seen,  that  if  (as  in  bell 
magnets,  horseshoe  magnets)  the  winding  is  not  carried  on 
right  round  the  bend  or  "  yoke,"  but  the  wire  made  to  cross 
over  to  the  other  limb,  the  winding  will  apparently  be  in  the 
opposite  sense,  that  is,  if  viewed  from  the  poles,  will  pass 
over  one  core  to  the  other,  thus  :  CD.  Lastly,  it  is  some- 
times necessary,  as  in  the  Gramme  machine,  §  72  Fig  43, 
to  produce  one  pole  in  the  middle  of  a  bar,  and  two  opposite 
poles  at  the  extremities.  In  a  Gramme  machine,  the  upper 
ole-piece  is  to  be,  say  north.  Hence,  the  two  extrmities 


APPENDIX.  199 

of  the  upper  bar  or  core  must  be  south.  To  obtain  this 
result  (consecutive  poles)  the  direction  of  winding  must  be 
reversed  when  we  cross  over  from  the  left-hand  bar  or  core, 
to  the  right-hand  one.  Suppose  we  start  winding  to  the 
loft-hand  of  the  upper  pole-piece,  and  begin  winding  in  a 
direction  Apposite  to  the  motion  of  clock-hands.  When  we 
have  coiled  the  bar  with  the  desired  amount  of  wire  (always 
in  the  same  direction)  we  carry  the  wire  over  the  pole-piece 
and  commence  winding  the  right-hand  core  or  bar  in  the 
opposite  direction,  ;',«.,  the  same  as  the  motion  of  the  hands 
of  a  clock. 


INDEX. 


Accumulators,  see  Appendix 
AJloy  for  Thermopile,  143 
Amalgamation,  148 
Ammeter,  116 
Ampere,  116 
Apparatus  necessary,  1 
Armature,  Insulation  of,  103 

Pacinotti,  109 

-^— ^—  Siemens,  99 
Winding,  102,  ill 


Batteries,  146,  189 

Chromic  Acid,  151 

Daniell,  154 

Double  fluid,  153 

— — —  Secondary,  Appendix 

Single  fluid,  149 

Table  of  E.M.F.,  &c.,  155 

Battery  for  coils,  88 
Bertsch's  Machine,  28 
Bichromated  Paste,  35 
Binding  Screws  (substitute^),  148 
Bobbins,  winding,  96 
Boxes,  G'ass-capped,  117 
Brass  Pivots,  9 
Brushes,  Wimshurst,  6$ 
Dynamo,  106  and  116 


Canada  Bajsam,  56 

Carre's  Dielectric  Machine,  49 

Cells,  140 

Creeping  in,  153 


Clarnond's  Thermopile,  143 
Coating  LeyttSn  Jar,  72 
Coils,  Induction,  88 

Medical,  81 

Primary,  83 

Secondary,  85 

Collectors,  69 
Combs,  49  and  55 
Commutator,  IOI,  Il6 
Condensers,  70 

Fizeau's,  76 

Microfarad,  79 

Consecutive  Poles,  115 
Contact  Breaker,  for  coil,  £4 

for  magneto,  98 


Core,  iron,  83 

Coulomb's  Torsion  balance,  20 

Creeping,  in  cells,  to  prevent,  153 

Cushions,  66 

Cutting  Zincs,  148 

Dynamic  Instrument.  8t 
Dynamo  Brushes,  1 06  and  I  ft 

Gramme,  loS 

Manchester,  109 

Magneto,  99 

•  Volts  in  a,  no 

E.M.F.,  what  it  is,  121 
Electro-motors,  164 

magnets,  197 

Electrophorus,  Volta's,  23 

•  Composition  for,  27 
Electroscopes,  pith  ball,  12 
Gold  leaf,  14 


INDEX. 


Faraday,  90 
Fizeau's  Condenser,  76 
Foucault's  Currents,  116 
Franklin's  Plate,  75 
Frictional  Instruments,  I 
Fulminating  Panes,  75 


Galvanometers,  131,  184. 

Astatic,  133 

Deflections,  table  of,  137 

Single  needle,  132 

Tangent,  134 

Glass-capped  boxes,  117 

Cutting,  29 

'• Drilling  holes  in,  31 

Plate,  mounting,  29  and  34 

Pivots,  9 

Threads,  II 

Testing  for    insulation,  41 

and  71 
Glue,  acetic,  32 

; —  Prout's  Elastic,  3 

Gold-leaf  electroscope,  14 
Gold-leaf,  to  cut,  18 
Grading,  119 
Graphite,  mounting,  150 


Hochhausen  Commutator,  116 
Holtz  Machine,  47 
Hubs,  condemned,  116 

Induction  Coils,  88 

Inductor,  54 

Influence  Machine  (Wimshurst),  57 

{Holtz),  47 

(Carre),  49 

(Bertsch),  28 

—  (Volta),  23 
Iron,  core,  83 
Iron,  soft,  importance  of,  115 


Jar,  spangle,  73 

Kay's  Coaguline,  receipt  for,  33 


Knobs  or  Balls,  size  of,  63 

Laminated  Armatures,  109 
Leyden  Jar,  71 


Magnetic  Needles,  to  make,  7  and 

118 
Magneto-electric  Machine,  90 

for  shocks,  90 

unidirection,  99 

Materials  indispensable,  2 
Microfarad  Condenser,  79 
Microphone,  1 80 
Mounting  Graphite  Rods  or  Plates, 

150 

Neutralising  Rods,  64 
Ohm's  Law,  122 

Pacinotti  Armature,  109 
Paper,  paraffined,  77 
Papyroxyline,  26 
Paraffin,  18 
Paraffined  Paper,  77 
Paste,  bichromated,  35 
Phonograph,  170 
Pith  Balls,  n 
Pivots,  brass,  9 

glass,  IO 

Pointers,  II,  129 
Poles,  consecutive,  IIJ 
Porous  Cells,  147 
Primary  Coil,  83 
Punchings  for  armature,  109 

Relation  of  current  to  defleclon  137 
Red  Varnish,  13,  37 

Resistance  to  armature  coils  112 
Table  of  wire,   112,  183 


Sal-ammoniac,   6,  155 

Sector,  59 

Shellac  Varnish,  to  mak«,  0B 


INDEX, 


Solder,  4 
Soldering,  3 

fluids,  5 

iron,  to  make,  3 

with  flame,  7 

Spangle  Jar,  73 
Spider  Wheel,  109 
Stands,  37,  42,  47,  60,  66 
Star  Wheel,  109 
Straws,  for  pointers,  &c.,  II 


Teaching  appliances,  Appendix 
Telephone,  156 
Thermopile,  139 

Alloy,  143 

Simple,  140 

Clamond's,  143 

Tinning,  a  soldering  iron,  5 
Tools,  needful,  i 
Triblet  Tubing,  81 
Tube,  glass,  to  work,  10 
Triblet,  81 


Varnish,  red,  13,  37 

Shellac,  58 

White,  hard,  13 

Volt,  no,  130 

Volta's  Electrophorus,  23 

Composition,  27 

Watt,  no 

Wheatstone's  Bridge  13$ 
Winter's  Ring,  69 
Wire  on  armature,  113 

On  Field  Magnets,  113 

Table  of  carrying  power,  164 

Table  of  resistance  of,  112 

testing  for  insulation,  103 

German  Silver,  125 

Zincs,  amalgamation  of,  148 
to  cut,  148 


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